ClickHouse Go

A simple example

Let's Go with a simple example. This will connect to ClickHouse and select from the system database. To get started you will need your connection details.

Connection Details

To connect to ClickHouse with native TCP you need this information:

• The HOST and PORT: typically, the port is 9440 when using TLS, or 9000 when not using TLS.

• The DATABASE NAME: out of the box there is a database named default, use the name of the database that you want to connect to.

• The USERNAME and PASSWORD: out of the box the username is default. Use the username appropriate for your use case.

The details for your ClickHouse Cloud service are available in the ClickHouse Cloud console. Select the service that you will connect to and click Connect:

Choose Native, and the details are available in an example clickhouse-client command.

If you are using self-managed ClickHouse, the connection details are set by your ClickHouse administrator.

Initialize a module

mkdir clickhouse-golang-example
cd clickhouse-golang-example
go mod init clickhouse-golang-example

Copy in some sample code

Copy this code into the clickhouse-golang-example directory as main.go.

main.go

package main

import (
    "context"
    "crypto/tls"
    "fmt"
    "log"

    "github.com/ClickHouse/clickhouse-go/v2"
    "github.com/ClickHouse/clickhouse-go/v2/lib/driver"
)

func main() {
    conn, err := connect()
    if err != nil {
        panic((err))
    }

    ctx := context.Background()
    rows, err := conn.Query(ctx, "SELECT name,toString(uuid) as uuid_str FROM system.tables LIMIT 5")
    if err != nil {
        log.Fatal(err)
    }

    for rows.Next() {
        var (
            name, uuid string
        )
        if err := rows.Scan(
            &name,
            &uuid,
        ); err != nil {
            log.Fatal(err)
        }
        log.Printf("name: %s, uuid: %s",
            name, uuid)
    }

}

func connect() (driver.Conn, error) {
    var (
        ctx       = context.Background()
        conn, err = clickhouse.Open(&clickhouse.Options{
            Addr: []string{"<CLICKHOUSE_SECURE_NATIVE_HOSTNAME>:9440"},
            Auth: clickhouse.Auth{
                Database: "default",
                Username: "default",
                Password: "<DEFAULT_USER_PASSWORD>",
            },
            ClientInfo: clickhouse.ClientInfo{
                Products: []struct {
                    Name    string
                    Version string
                }{
                    {Name: "an-example-go-client", Version: "0.1"},
                },
            },

            Debugf: func(format string, v ...interface{}) {
                fmt.Printf(format, v)
            },
            TLS: &tls.Config{
                InsecureSkipVerify: true,
            },
        })
    )

    if err != nil {
        return nil, err
    }

    if err := conn.Ping(ctx); err != nil {
        if exception, ok := err.(*clickhouse.Exception); ok {
            fmt.Printf("Exception [%d] %s \n%s\n", exception.Code, exception.Message, exception.StackTrace)
        }
        return nil, err
    }
    return conn, nil
}

Run go mod tidy

go mod tidy

Set your connection details

Earlier you looked up your connection details. Set them in main.go in the connect() function:

func connect() (driver.Conn, error) {
  var (
    ctx       = context.Background()
    conn, err = clickhouse.Open(&clickhouse.Options{
      Addr: []string{"<CLICKHOUSE_SECURE_NATIVE_HOSTNAME>:9440"},
      Auth: clickhouse.Auth{
        Database: "default",
        Username: "default",
        Password: "<DEFAULT_USER_PASSWORD>",
      },

Run the example

go run .

2023/03/06 14:18:33 name: COLUMNS, uuid: 00000000-0000-0000-0000-000000000000
2023/03/06 14:18:33 name: SCHEMATA, uuid: 00000000-0000-0000-0000-000000000000
2023/03/06 14:18:33 name: TABLES, uuid: 00000000-0000-0000-0000-000000000000
2023/03/06 14:18:33 name: VIEWS, uuid: 00000000-0000-0000-0000-000000000000
2023/03/06 14:18:33 name: hourly_data, uuid: a4e36bd4-1e82-45b3-be77-74a0fe65c52b

Learn more

The rest of the documentation in this category covers the details of the ClickHouse Go client.

ClickHouse Go Client

ClickHouse supports two official Go clients. These clients are complementary and intentionally support different use cases.

• clickhouse-go - High level language client which supports either the Go standard database/sql interface or the native interface.
• ch-go - Low level client. Native interface only.

clickhouse-go provides a high-level interface, allowing users to query and insert data using row-orientated semantics and batching that are lenient with respect to data types - values will be converted provided no precision loss is potentially incurred. ch-go, meanwhile, provides an optimized column-orientated interface that provides fast data block streaming with low CPU and memory overhead at the expense of type strictness and more complex usage.

From version 2.3, Clickhouse-go utilizes ch-go for low-level functions such as encoding, decoding, and compression. Note that clickhouse-go also supports the Go database/sql interface standard. Both clients use the native format for their encoding to provide optimal performance and can communicate over the native ClickHouse protocol. clickhouse-go also supports HTTP as its transport mechanism for cases where users have a requirement to proxy or load balance traffic.

When choosing a client library, users should be aware of their respective pros and cons - see Choosing a Client Library.

	Native format	Native protocol	HTTP protocol	Row Orientated API	Column Orientated API	Type flexibility	Compression	Query Placeholders
clickhouse-go	✅	✅	✅	✅	✅	✅	✅	✅
ch-go	✅	✅			✅		✅

Choosing a Client

Selecting a client library depends on your usage patterns and need for optimal performance. For insert heavy use cases, where millions of inserts are required per second, we recommend using the low level client ch-go. This client avoids the associated overhead of pivoting the data from a row-orientated format to columns, as the ClickHouse native format requires. Furthermore, it avoids any reflection or use of the interface{} (any) type to simplify usage.

For query workloads focused on aggregations or lower throughput insert workloads, the clickhouse-go provides a familiar database/sql interface and more straightforward row semantics. Users can also optionally use HTTP for the transport protocol and take advantage of helper functions to marshal rows to and from structs.

The clickhouse-go Client

The clickhouse-go client provides two API interfaces for communicating with ClickHouse:

• ClickHouse client-specific API
• database/sql standard - generic interface around SQL databases provided by Golang.

While the database/sql provides a database-agnostic interface, allowing developers to abstract their data store, it enforces some typing and query semantics that impact performance. For this reason, the client-specific API should be used where performance is important. However, users who wish to integrate ClickHouse into tooling, which supports multiple databases, may prefer to use the standard interface.

Both interfaces encode data using the native format and native protocol for communication. Additionally, the standard interface supports communication over HTTP.

	Native format	Native protocol	HTTP protocol	Bulk write support	Struct marshaling	Compression	Query Placeholders
ClickHouse API	✅	✅		✅	✅	✅	✅
database/sql API	✅	✅	✅	✅		✅	✅

Installation

v1 of the driver is deprecated and will not reach feature updates or support for new ClickHouse types. Users should migrate to v2, which offers superior performance.

To install the 2.x version of the client, add the package to your go.mod file:

require github.com/ClickHouse/clickhouse-go/v2 main

Or, clone the repository:

git clone --branch v2 https://github.com/clickhouse/clickhouse-go.git $GOPATH/src/github

To install another version, modify the path or the branch name accordingly.

mkdir my-clickhouse-app && cd my-clickhouse-app

cat > go.mod <<-END
  module my-clickhouse-app

  go 1.18

  require github.com/ClickHouse/clickhouse-go/v2 main
END

cat > main.go <<-END
  package main

  import (
    "fmt"
    "github.com/ClickHouse/clickhouse-go/v2"
  )

  func main() {
   conn, _ := clickhouse.Open(&clickhouse.Options{Addr: []string{"127.0.0.1:9000"}})
    v, _ := conn.ServerVersion()
    fmt.Println(v.String())
  }
END

go mod tidy
go run main.go

Versioning & compatibility

The client is released independently of ClickHouse. 2.x represents the current major under development. All versions of 2.x should be compatibile with each other.

ClickHouse compatibility

The client supports:

• All currently supported versions of ClickHouse as recorded here. As ClickHouse versions are no longer supported they are also no longer actively tested against client releases.
• All versions of ClickHouse 2 years from the release date of the client. Note only LTS versions are actively tested.

Golang compatibility

Client Version	Golang Versions
=> 2.0 <= 2.2	1.17, 1.18
>= 2.3	1.18

ClickHouse Client API

All code examples for the ClickHouse Client API can be found here.

Connecting

The following example, which returns the server version, demonstrates connecting to ClickHouse - assuming ClickHouse is not secured and accessible with the default user.

Note we use the default native port to connect.

conn, err := clickhouse.Open(&clickhouse.Options{
    Addr: []string{fmt.Sprintf("%s:%d", env.Host, env.Port)},
    Auth: clickhouse.Auth{
        Database: env.Database,
        Username: env.Username,
        Password: env.Password,
    },
})
if err != nil {
    return err
}
v, err := conn.ServerVersion()
fmt.Println(v)

Full Example

For all subsequent examples, unless explicitly shown, we assume the use of the ClickHouse conn variable has been created and is available.

Connection Settings

When opening a connection, an Options struct can be used to control client behavior. The following settings are available:

• Protocol - either Native or HTTP. HTTP is only supported currently for the database/sql API.
• TLS - TLS options. A non-nil value enables TLS. See Using TLS.
• Addr - a slice of addresses including port.
• Auth - Authentication detail. See Authentication.
• DialContext - custom dial function to determine how connections are established.
• Debug - true/false to enable debugging.
• Debugf - provides a function to consume debug output. Requires debug to be set to true.
• Settings - map of ClickHouse settings. These will be applied to all ClickHouse queries. Using Context allows settings to be set per query.
• Compression - enable compression for blocks. See Compression.
• DialTimeout - the maximum time to establish a connection. Defaults to 1s.
• MaxOpenConns - max connections for use at any time. More or fewer connections may be in the idle pool, but only this number can be used at any time. Defaults to MaxIdleConns+5.
• MaxIdleConns - number of connections to maintain in the pool. Connections will be reused if possible. Defaults to 5.
• ConnMaxLifetime - maximum lifetime to keep a connection available. Defaults to 1hr. Connections are destroyed after this time, with new connections added to the pool as required.
• ConnOpenStrategy - determines how the list of node addresses should be consumed and used to open connections. See Connecting to Multiple Nodes.
• BlockBufferSize - maximum number of blocks to decode into the buffer at once. Larger values will increase parallelization at the expense of memory. Block sizes are query dependent so while you can set this on the connection, we recommend you override per query based on the data it returns. Defaults to 2.

conn, err := clickhouse.Open(&clickhouse.Options{
    Addr: []string{fmt.Sprintf("%s:%d", env.Host, env.Port)},
    Auth: clickhouse.Auth{
        Database: env.Database,
        Username: env.Username,
        Password: env.Password,
    },
    DialContext: func(ctx context.Context, addr string) (net.Conn, error) {
        dialCount++
        var d net.Dialer
        return d.DialContext(ctx, "tcp", addr)
    },
    Debug: true,
    Debugf: func(format string, v ...interface{}) {
        fmt.Printf(format, v)
    },
    Settings: clickhouse.Settings{
        "max_execution_time": 60,
    },
    Compression: &clickhouse.Compression{
        Method: clickhouse.CompressionLZ4,
    },
    DialTimeout:      time.Duration(10) * time.Second,
    MaxOpenConns:     5,
    MaxIdleConns:     5,
    ConnMaxLifetime:  time.Duration(10) * time.Minute,
    ConnOpenStrategy: clickhouse.ConnOpenInOrder,
    BlockBufferSize: 10,
})
if err != nil {
    return err
}

Full Example

Connection Pooling

The client maintains a pool of connections, reusing these across queries as required. At most, MaxOpenConns will be used at any time, with the maximum pool size controlled by the MaxIdleConns. The client will acquire a connection from the pool for each query execution, returning it to the pool for reuse. A connection is used for the lifetime of a batch and released on Send().

There is no guarantee the same connection in a pool will be used for subsequent queries unless the user sets MaxOpenConns=1. This is rarely needed but may be required for cases where users are using temporary tables.

Also, note that the ConnMaxLifetime is by default 1hr. This can lead to cases where the load to ClickHouse becomes unbalanced if nodes leave the cluster. This can occur when a node becomes unavailable, connections will balance to the other nodes. These connections will persist and not be refreshed for 1hr by default, even if the problematic node returns to the cluster. Consider lowering this value in heavy workload cases.

Using TLS

At a low level, all client connect methods (DSN/OpenDB/Open) will use the Go tls package to establish a secure connection. The client knows to use TLS if the Options struct contains a non-nil tls.Config pointer.

env, err := GetNativeTestEnvironment()
if err != nil {
    return err
}
cwd, err := os.Getwd()
if err != nil {
    return err
}
t := &tls.Config{}
caCert, err := ioutil.ReadFile(path.Join(cwd, "../../tests/resources/CAroot.crt"))
if err != nil {
    return err
}
caCertPool := x509.NewCertPool()
successful := caCertPool.AppendCertsFromPEM(caCert)
if !successful {
    return err
}
t.RootCAs = caCertPool
conn, err := clickhouse.Open(&clickhouse.Options{
    Addr: []string{fmt.Sprintf("%s:%d", env.Host, env.SslPort)},
    Auth: clickhouse.Auth{
        Database: env.Database,
        Username: env.Username,
        Password: env.Password,
    },
    TLS: t,
})
if err != nil {
    return err
}
v, err := conn.ServerVersion()
if err != nil {
    return err
}
fmt.Println(v.String())

Full Example

This minimal TLS.Config is normally sufficient to connect to the secure native port (normally 9440) on a ClickHouse server. If the ClickHouse server does not have a valid certificate (expired, wrong hostname, not signed by a publicly recognized root Certificate Authority), InsecureSkipVerify can be true, but this is strongly discouraged.

conn, err := clickhouse.Open(&clickhouse.Options{
    Addr: []string{fmt.Sprintf("%s:%d", env.Host, env.SslPort)},
    Auth: clickhouse.Auth{
        Database: env.Database,
        Username: env.Username,
        Password: env.Password,
    },
    TLS: &tls.Config{
        InsecureSkipVerify: true,
    },
})
if err != nil {
    return err
}
v, err := conn.ServerVersion()

Full Example

If additional TLS parameters are necessary, the application code should set the desired fields in the tls.Config struct. That can include specific cipher suites, forcing a particular TLS version (like 1.2 or 1.3), adding an internal CA certificate chain, adding a client certificate (and private key) if required by the ClickHouse server, and most of the other options that come with a more specialized security setup.

Authentication

Specify an Auth struct in the connection details to specify a username and password.

conn, err := clickhouse.Open(&clickhouse.Options{
    Addr: []string{fmt.Sprintf("%s:%d", env.Host, env.Port)},
    Auth: clickhouse.Auth{
        Database: env.Database,
        Username: env.Username,
        Password: env.Password,
    },
})
if err != nil {
    return err
}
if err != nil {
    return err
}
v, err := conn.ServerVersion()

Full Example

Connecting to Multiple Nodes

Multiple addresses can be specified via the Addr struct.

conn, err := clickhouse.Open(&clickhouse.Options{
    Addr: []string{"127.0.0.1:9001", "127.0.0.1:9002", fmt.Sprintf("%s:%d", env.Host, env.Port)},
    Auth: clickhouse.Auth{
        Database: env.Database,
        Username: env.Username,
        Password: env.Password,
    },
})
if err != nil {
    return err
}
v, err := conn.ServerVersion()
if err != nil {
    return err
}
fmt.Println(v.String())

Full Example

Two connection strategies are available:

• ConnOpenInOrder (default) - addresses are consumed in order. Later addresses are only utilized in case of failure to connect using addresses earlier in the list. This is effectively a failure-over strategy.
• ConnOpenRoundRobin - Load is balanced across the addresses using a round-robin strategy.

This can be controlled through the option ConnOpenStrategy

conn, err := clickhouse.Open(&clickhouse.Options{
    Addr:             []string{"127.0.0.1:9001", "127.0.0.1:9002", fmt.Sprintf("%s:%d", env.Host, env.Port)},
    ConnOpenStrategy: clickhouse.ConnOpenRoundRobin,
    Auth: clickhouse.Auth{
        Database: env.Database,
        Username: env.Username,
        Password: env.Password,
    },
})
if err != nil {
    return err
}
v, err := conn.ServerVersion()
if err != nil {
    return err
}

Full Example

Execution

Arbitrary statements can be executed via the Exec method. This is useful for DDL and simple statements. It should not be used for larger inserts or query iterations.

conn.Exec(context.Background(), `DROP TABLE IF EXISTS example`)
err = conn.Exec(context.Background(), `
    CREATE TABLE IF NOT EXISTS example (
        Col1 UInt8,
        Col2 String
    ) engine=Memory
`)
if err != nil {
    return err
}
conn.Exec(context.Background(), "INSERT INTO example VALUES (1, 'test-1')")

Full Example

Note the ability to pass a Context to the query. This can be used to pass specific query level settings - see Using Context.

Batch Insert

To insert a large number of rows, the client provides batch semantics. This requires the preparation of a batch to which rows can be appended. This is finally sent via the Send() method. Batches will be held in memory until Send is executed.

conn, err := GetNativeConnection(nil, nil, nil)
if err != nil {
    return err
}
ctx := context.Background()
defer func() {
    conn.Exec(ctx, "DROP TABLE example")
}()
conn.Exec(context.Background(), "DROP TABLE IF EXISTS example")
err = conn.Exec(ctx, `
    CREATE TABLE IF NOT EXISTS example (
            Col1 UInt8
        , Col2 String
        , Col3 FixedString(3)
        , Col4 UUID
        , Col5 Map(String, UInt8)
        , Col6 Array(String)
        , Col7 Tuple(String, UInt8, Array(Map(String, String)))
        , Col8 DateTime
    ) Engine = Memory
`)
if err != nil {
    return err
}


batch, err := conn.PrepareBatch(ctx, "INSERT INTO example")
if err != nil {
    return err
}
for i := 0; i < 1000; i++ {
    err := batch.Append(
        uint8(42),
        "ClickHouse",
        "Inc",
        uuid.New(),
        map[string]uint8{"key": 1},             // Map(String, UInt8)
        []string{"Q", "W", "E", "R", "T", "Y"}, // Array(String)
        []interface{}{ // Tuple(String, UInt8, Array(Map(String, String)))
            "String Value", uint8(5), []map[string]string{
                {"key": "value"},
                {"key": "value"},
                {"key": "value"},
            },
        },
        time.Now(),
    )
    if err != nil {
        return err
    }
}
return batch.Send()

Full Example

Recommendations for ClickHouse apply here. Batches should not be shared across go-routines - construct a separate batch per routine.

From the above example, note the need for variable types to align with the column type when appending rows. While the mapping is usually obvious, this interface tries to be flexible, and types will be converted provided no precision loss is incurred. For example, the following demonstrates inserting a string into a datetime64.

batch, err := conn.PrepareBatch(ctx, "INSERT INTO example")
if err != nil {
    return err
}
for i := 0; i < 1000; i++ {
    err := batch.Append(
        "2006-01-02 15:04:05.999",
    )
    if err != nil {
        return err
    }
}
return batch.Send()

Full Example

For a full summary of supported go types for each column type, see Type Conversions.

Querying Row/s

Users can either query for a single row using the QueryRow method or obtain a cursor for iteration over a result set via Query. While the former accepts a destination for the data to be serialized into, the latter requires the to call Scan on each row.

row := conn.QueryRow(context.Background(), "SELECT * FROM example")
var (
    col1             uint8
    col2, col3, col4 string
    col5             map[string]uint8
    col6             []string
    col7             []interface{}
    col8             time.Time
)
if err := row.Scan(&col1, &col2, &col3, &col4, &col5, &col6, &col7, &col8); err != nil {
    return err
}
fmt.Printf("row: col1=%d, col2=%s, col3=%s, col4=%s, col5=%v, col6=%v, col7=%v, col8=%v\n", col1, col2, col3, col4, col5, col6, col7, col8)

Full Example

rows, err := conn.Query(ctx, "SELECT Col1, Col2, Col3 FROM example WHERE Col1 >= 2")
if err != nil {
    return err
}
for rows.Next() {
    var (
        col1 uint8
        col2 string
        col3 time.Time
    )
    if err := rows.Scan(&col1, &col2, &col3); err != nil {
        return err
    }
    fmt.Printf("row: col1=%d, col2=%s, col3=%s\n", col1, col2, col3)
}
rows.Close()
return rows.Err()

Full Example

Note in both cases, we are required to pass a pointer to the variables we wish to serialize the respective column values into. These must be passed in the order specified in the SELECT statement - by default, the order of column declaration will be used in the event of a SELECT * as shown above.

Similar to insertion, the Scan method requires the target variables to be of an appropriate type. This again aims to be flexible, with types converted where possible, provided no precision loss is possible, e.g., the above example shows a UUID column being read into a string variable. For a full list of supported go types for each Column type, see Type Conversions.

Finally, note the ability to pass a Context to the Query and QueryRow methods. This can be used for query level settings - see Using Context for further details.

Async Insert

Asynchronous inserts are supported through the Async method. This allows the user to specify whether the client should wait for the server to complete the insert or respond once the data has been received. This effectively controls the parameter wait_for_async_insert.

conn, err := GetNativeConnection(nil, nil, nil)
if err != nil {
    return err
}
ctx := context.Background()
if err := clickhouse_tests.CheckMinServerServerVersion(conn, 21, 12, 0); err != nil {
    return nil
}
defer func() {
    conn.Exec(ctx, "DROP TABLE example")
}()
conn.Exec(ctx, `DROP TABLE IF EXISTS example`)
const ddl = `
    CREATE TABLE example (
            Col1 UInt64
        , Col2 String
        , Col3 Array(UInt8)
        , Col4 DateTime
    ) ENGINE = Memory
`
if err := conn.Exec(ctx, ddl); err != nil {
    return err
}
for i := 0; i < 100; i++ {
    if err := conn.AsyncInsert(ctx, fmt.Sprintf(`INSERT INTO example VALUES (
        %d, '%s', [1, 2, 3, 4, 5, 6, 7, 8, 9], now()
    )`, i, "Golang SQL database driver"), false); err != nil {
        return err
    }
}

Full Example

Columnar Insert

Inserts can be inserted in column format. This can provide performance benefits if the data is already orientated in this structure by avoiding the need to pivot to rows.

batch, err := conn.PrepareBatch(context.Background(), "INSERT INTO example")
if err != nil {
    return err
}
var (
    col1 []uint64
    col2 []string
    col3 [][]uint8
    col4 []time.Time
)
for i := 0; i < 1_000; i++ {
    col1 = append(col1, uint64(i))
    col2 = append(col2, "Golang SQL database driver")
    col3 = append(col3, []uint8{1, 2, 3, 4, 5, 6, 7, 8, 9})
    col4 = append(col4, time.Now())
}
if err := batch.Column(0).Append(col1); err != nil {
    return err
}
if err := batch.Column(1).Append(col2); err != nil {
    return err
}
if err := batch.Column(2).Append(col3); err != nil {
    return err
}
if err := batch.Column(3).Append(col4); err != nil {
    return err
}
return batch.Send()

Full Example

Using Structs

For users, Golang structs provide a logical representation of a row of data in ClickHouse. To assist with this, the native interface provides several convenient functions.

Select with Serialize

The Select method allows a set of response rows to be marshaled into a slice of structs with a single invocation.

var result []struct {
    Col1           uint8
    Col2           string
    ColumnWithName time.Time `ch:"Col3"`
}

if err = conn.Select(ctx, &result, "SELECT Col1, Col2, Col3 FROM example"); err != nil {
    return err
}

for _, v := range result {
    fmt.Printf("row: col1=%d, col2=%s, col3=%s\n", v.Col1, v.Col2, v.ColumnWithName)
}

Full Example

Scan Struct

ScanStruct allows the marshaling of a single Row from a query into a struct.

var result struct {
    Col1  int64
    Count uint64 `ch:"count"`
}
if err := conn.QueryRow(context.Background(), "SELECT Col1, COUNT() AS count FROM example WHERE Col1 = 5 GROUP BY Col1").ScanStruct(&result); err != nil {
    return err
}

Full Example

Append Struct

AppendStruct allows a struct to be appended to an existing batch and interpreted as a complete row. This requires the columns of the struct to align in both name and type with the table. While all columns must have an equivalent struct field, some struct fields may not have an equivalent column representation. These will simply be ignored.

batch, err := conn.PrepareBatch(context.Background(), "INSERT INTO example")
if err != nil {
    return err
}
for i := 0; i < 1_000; i++ {
    err := batch.AppendStruct(&row{
        Col1:       uint64(i),
        Col2:       "Golang SQL database driver",
        Col3:       []uint8{1, 2, 3, 4, 5, 6, 7, 8, 9},
        Col4:       time.Now(),
        ColIgnored: "this will be ignored",
    })
    if err != nil {
        return err
    }
}

Full Example

Type Conversions

The client aims to be as flexible as possible concerning accepting variable types for both insertion and marshaling of responses. In most cases, an equivalent Golang type exists for a ClickHouse column type, e.g., UInt64 to uint64. These logical mappings should always be supported. Users may wish to utilize variable types that can be inserted into columns or used to receive a response if the conversion of either the variable or received data takes place first. The client aims to support these conversions transparently, so users do not need to convert their data to align precisely before insertion and to provide flexible marshaling at query time. This transparent conversion does not allow for precision loss. For example, a uint32 cannot be used to receive data from a UInt64 column. Conversely, a string can be inserted into a datetime64 field provided it meets the format requirements.

The type conversions currently supported for primitive types are captured here.

This effort is ongoing and can be separated into insertion (Append/AppendRow) and read time (via a Scan). Should you need support for a specific conversion, please raise an issue.

Complex Types

Date/DateTime types

The ClickHouse go client supports the Date, Date32, DateTime, and DateTime64 date/datetime types. Dates can be inserted as a string in the format 2006-01-02 or using the native go time.Time{} or sql.NullTime. DateTimes also support the latter types but require strings to be passed in the format 2006-01-02 15:04:05 with an optional timezone offset e.g. 2006-01-02 15:04:05 +08:00. time.Time{} and sql.NullTime are both supported at read time as well as any implementation of of the sql.Scanner interface.

Handling of timezone information depends on the ClickHouse type and whether the value is being inserted or read:

• DateTime/DateTime64
  • At insert time the value is sent to ClickHouse in UNIX timestamp format. If no time zone is provided, the client will assume the client's local time zone. time.Time{} or sql.NullTime will be converted to epoch accordingly.
  • At select time the timezone of the column will be used if set when returning a time.Time value. If not, the timezone of the server will be used.
• Date/Date32
  • At insert time, the timezone of any date is considered when converting the date to a unix timestamp, i.e., it will be offset by the timezone prior to storage as a date, as Date types have no locale in ClickHouse. If this is not specified in a string value, the local timezone will be used.
  • At select time, dates are scanned into time.Time{} or sql.NullTime{} instances will be returned without timezone information.

Array

Arrays should be inserted as slices. Typing rules for the elements are consistent with those for the primitive type, i.e., where possible elements will be converted.

A pointer to a slice should be provided at Scan time.

batch, err := conn.PrepareBatch(ctx, "INSERT INTO example")
if err != nil {
    return err
}
var i int64
for i = 0; i < 10; i++ {
    err := batch.Append(
        []string{strconv.Itoa(int(i)), strconv.Itoa(int(i + 1)), strconv.Itoa(int(i + 2)), strconv.Itoa(int(i + 3))},
        [][]int64{{i, i + 1}, {i + 2, i + 3}, {i + 4, i + 5}},
    )
    if err != nil {
        return err
    }
}
if err := batch.Send(); err != nil {
    return err
}
var (
    col1 []string
    col2 [][]int64
)
rows, err := conn.Query(ctx, "SELECT * FROM example")
if err != nil {
    return err
}
for rows.Next() {
    if err := rows.Scan(&col1, &col2); err != nil {
        return err
    }
    fmt.Printf("row: col1=%v, col2=%v\n", col1, col2)
}
rows.Close()

Full Example

Map

Maps should be inserted as Golang maps with keys and values conforming to the type rules defined earlier.

batch, err := conn.PrepareBatch(ctx, "INSERT INTO example")
if err != nil {
    return err
}
var i int64
for i = 0; i < 10; i++ {
    err := batch.Append(
        map[string]uint64{strconv.Itoa(int(i)): uint64(i)},
        map[string][]string{strconv.Itoa(int(i)): {strconv.Itoa(int(i)), strconv.Itoa(int(i + 1)), strconv.Itoa(int(i + 2)), strconv.Itoa(int(i + 3))}},
        map[string]map[string]uint64{strconv.Itoa(int(i)): {strconv.Itoa(int(i)): uint64(i)}},
    )
    if err != nil {
        return err
    }
}
if err := batch.Send(); err != nil {
    return err
}
var (
    col1 map[string]uint64
    col2 map[string][]string
    col3 map[string]map[string]uint64
)
rows, err := conn.Query(ctx, "SELECT * FROM example")
if err != nil {
    return err
}
for rows.Next() {
    if err := rows.Scan(&col1, &col2, &col3); err != nil {
        return err
    }
    fmt.Printf("row: col1=%v, col2=%v, col3=%v\n", col1, col2, col3)
}
rows.Close()

Full Example

Tuples

Tuples represent a group of Columns of arbitrary length. The columns can either be explicitly named or only specify a type e.g.

//unnamed
Col1 Tuple(String, Int64)

//named
Col2 Tuple(name String, id Int64, age uint8)

Of these approaches, named tuples offer greater flexibility. While unnamed tuples must be inserted and read using slices, named tuples are also compatible with maps.

if err = conn.Exec(ctx, `
    CREATE TABLE example (
            Col1 Tuple(name String, age UInt8),
            Col2 Tuple(String, UInt8),
            Col3 Tuple(name String, id String)
        )
        Engine Memory
    `); err != nil {
    return err
}

defer func() {
    conn.Exec(ctx, "DROP TABLE example")
}()
batch, err := conn.PrepareBatch(ctx, "INSERT INTO example")
if err != nil {
    return err
}
// both named and unnamed can be added with slices. Note we can use strongly typed lists and maps if all elements are the same type
if err = batch.Append([]interface{}{"Clicky McClickHouse", uint8(42)}, []interface{}{"Clicky McClickHouse Snr", uint8(78)}, []string{"Dale", "521211"}); err != nil {
    return err
}
if err = batch.Append(map[string]interface{}{"name": "Clicky McClickHouse Jnr", "age": uint8(20)}, []interface{}{"Baby Clicky McClickHouse", uint8(1)}, map[string]string{"name": "Geoff", "id": "12123"}); err != nil {
    return err
}
if err = batch.Send(); err != nil {
    return err
}
var (
    col1 map[string]interface{}
    col2 []interface{}
    col3 map[string]string
)
// named tuples can be retrieved into a map or slices, unnamed just slices
if err = conn.QueryRow(ctx, "SELECT * FROM example").Scan(&col1, &col2, &col3); err != nil {
    return err
}
fmt.Printf("row: col1=%v, col2=%v, col3=%v\n", col1, col2, col3)

Full Example

Note: typed slices and maps are supported, provide the sub-columns in the named tuple are all of the same types.

Nested

A Nested field is equivalent to an Array of named Tuples. Usage depends on whether the user has set flatten_nested to 1 or 0.

By setting flatten_nested to 0, Nested columns stay as a single array of tuples. This allows users to use slices of maps for insertion and retrieval and arbitrary levels of nesting. The map's key must equal the column's name, as shown in the example below.

Note: since the maps represent a tuple, they must be of the type map[string]interface{}. The values are currently not strongly typed.

conn, err := GetNativeConnection(clickhouse.Settings{
    "flatten_nested": 0,
}, nil, nil)
if err != nil {
    return err
}
ctx := context.Background()
defer func() {
    conn.Exec(ctx, "DROP TABLE example")
}()
conn.Exec(context.Background(), "DROP TABLE IF EXISTS example")
err = conn.Exec(ctx, `
    CREATE TABLE example (
        Col1 Nested(Col1_1 String, Col1_2 UInt8),
        Col2 Nested(
            Col2_1 UInt8,
            Col2_2 Nested(
                Col2_2_1 UInt8,
                Col2_2_2 UInt8
            )
        )
    ) Engine Memory
`)
if err != nil {
    return err
}

batch, err := conn.PrepareBatch(ctx, "INSERT INTO example")
if err != nil {
    return err
}
var i int64
for i = 0; i < 10; i++ {
    err := batch.Append(
        []map[string]interface{}{
            {
                "Col1_1": strconv.Itoa(int(i)),
                "Col1_2": uint8(i),
            },
            {
                "Col1_1": strconv.Itoa(int(i + 1)),
                "Col1_2": uint8(i + 1),
            },
            {
                "Col1_1": strconv.Itoa(int(i + 2)),
                "Col1_2": uint8(i + 2),
            },
        },
        []map[string]interface{}{
            {
                "Col2_2": []map[string]interface{}{
                    {
                        "Col2_2_1": uint8(i),
                        "Col2_2_2": uint8(i + 1),
                    },
                },
                "Col2_1": uint8(i),
            },
            {
                "Col2_2": []map[string]interface{}{
                    {
                        "Col2_2_1": uint8(i + 2),
                        "Col2_2_2": uint8(i + 3),
                    },
                },
                "Col2_1": uint8(i + 1),
            },
        },
    )
    if err != nil {
        return err
    }
}
if err := batch.Send(); err != nil {
    return err
}
var (
    col1 []map[string]interface{}
    col2 []map[string]interface{}
)
rows, err := conn.Query(ctx, "SELECT * FROM example")
if err != nil {
    return err
}
for rows.Next() {
    if err := rows.Scan(&col1, &col2); err != nil {
        return err
    }
    fmt.Printf("row: col1=%v, col2=%v\n", col1, col2)
}
rows.Close()

Full Example - flatten_tested=0

If the default value of 1 is used for flatten_nested, nested columns are flattened to separate arrays. This requires using nested slices for insertion and retrieval. While arbitrary levels of nesting may work, this is not officially supported.

conn, err := GetNativeConnection(nil, nil, nil)
if err != nil {
    return err
}
ctx := context.Background()
defer func() {
    conn.Exec(ctx, "DROP TABLE example")
}()
conn.Exec(ctx, "DROP TABLE IF EXISTS example")
err = conn.Exec(ctx, `
    CREATE TABLE example (
        Col1 Nested(Col1_1 String, Col1_2 UInt8),
        Col2 Nested(
            Col2_1 UInt8,
            Col2_2 Nested(
                Col2_2_1 UInt8,
                Col2_2_2 UInt8
            )
        )
    ) Engine Memory
`)
if err != nil {
    return err
}


batch, err := conn.PrepareBatch(ctx, "INSERT INTO example")
if err != nil {
    return err
}
var i uint8
for i = 0; i < 10; i++ {
    col1_1_data := []string{strconv.Itoa(int(i)), strconv.Itoa(int(i + 1)), strconv.Itoa(int(i + 2))}
    col1_2_data := []uint8{i, i + 1, i + 2}
    col2_1_data := []uint8{i, i + 1, i + 2}
    col2_2_data := [][][]interface{}{
        {
            {i, i + 1},
        },
        {
            {i + 2, i + 3},
        },
        {
            {i + 4, i + 5},
        },
    }
    err := batch.Append(
        col1_1_data,
        col1_2_data,
        col2_1_data,
        col2_2_data,
    )
    if err != nil {
        return err
    }
}
if err := batch.Send(); err != nil {
    return err
}

Full Example - flatten_nested=1

Note: Nested columns must have the same dimensions. For example, in the above example, Col_2_2 and Col_2_1 must have the same number of elements.

Due to a more straightforward interface and official support for nesting, we recommend flatten_nested=0.

JSON

The JSON type utilizes schema inference to automatically create arbitrary levels of tuples, nested objects, and arrays in order to represent JSON data. Other than a root JSON column, the user does not need to define any column types - these will be automatically inferred from the data and created as required. For further details, see Working with JSON.

This feature is only available in versions later than 22.3. It represents the preferred mechanism for handling arbitrary semi-structured JSON. To provide maximum flexibility, the go client allows JSON to be inserted using a struct, map, or string. JSON columns can be marshaled back into either a struct or map. Examples of each approach are shown below.

Note the need to set allow_experimental_object_type=1 since JSON is experimental.

if err = conn.Exec(ctx, `
    CREATE TABLE example (
            Col1 JSON,
            Col2 JSON,
            Col3 JSON
        )
        Engine Memory
    `); err != nil {
    return err
}


type User struct {
    Name     string `json:"name"`
    Age      uint8  `json:"age"`
    Password string `ch:"-"`
}


defer func() {
    conn.Exec(ctx, "DROP TABLE example")
}()
batch, err := conn.PrepareBatch(ctx, "INSERT INTO example")
if err != nil {
    return err
}
// we can insert JSON as either a string, struct or map
col1Data := `{"name": "Clicky McClickHouse", "age": 40, "password": "password"}`
col2Data := User{
    Name:     "Clicky McClickHouse Snr",
    Age:      uint8(80),
    Password: "random",
}
col3Data := map[string]interface{}{
    "name":     "Clicky McClickHouse Jnr",
    "age":      uint8(10),
    "password": "clicky",
}
// both named and unnamed can be added with slices
if err = batch.Append(col1Data, col2Data, col3Data); err != nil {
    return err
}

if err = batch.Send(); err != nil {
    return err
}
// we can scan JSON into either a map or struct
var (
    col1 map[string]interface{}
    col2 map[string]interface{}
    col3 User
)
// named tuples can be retrieved into a map or slices, unnamed just slices
if err = conn.QueryRow(ctx, "SELECT * FROM example").Scan(&col1, &col2, &col3); err != nil {
    return err
}

Full Example

Maps can be strongly typed. Maps and structs can also be nested in any combination. See the more complex example here:

If inserting structs, the client supports using json tags to control the name of a field when serialized. Fields can also be ignored if using the special value of - e.g.

type User struct {
  Name     string `json:"name"`
  Age      uint8  `json:"age"`
  Password string `json:"-"`
}

The “json” tag is respected by the standard golang json encoding package. The client also supports the “ch” tag equivalent to “json”. For example, the following is equivalent to the above for the client:

type User struct {
  Name     string `ch:"name"`
  Age      uint8  `ch:"age"`
  Password string `ch:"-"`
}

Important Notes

• No current support for structs and maps which have pointers inside them.
• If inserting structs or maps, types must be consistent within a batch, i.e., a field must be consistent within a batch. Note that ClickHouse is able to downcast/coerce types, e.g., if a field is an int and then sent as a String, the field will be converted to a String - see here for further examples. The client will not do this since it would require visibility of the complete dataset. If you need flexible types within a batch, insert the JSON as a string, e.g., you can't be sure if a JSON field is a string or number. This will defer type decisions and processing to the server.
• Within a batch, we don't allow mixed formats for the same column e.g. strings with maps/structs. Maps and structs are equivalent and can be added to the same column. The examples above use strings for a different JSON column.
• Dimensions and types within a slice must be the same e.g., an interface{} slice with a struct and list is not supported. This is the same behavior as ClickHouse, where lists of objects with different dimensions are not supported.
• At query time, we do best-effort filling structs passed. Any data for which there is no field is currently ignored.

Geo Types

The client supports the geo types Point, Ring, Polygon, and Multi Polygon. These fields are in Golang using the package github.com/paulmach/orb.

if err = conn.Exec(ctx, `
    CREATE TABLE example (
            point Point,
            ring Ring,
            polygon Polygon,
            mPolygon MultiPolygon
        )
        Engine Memory
    `); err != nil {
    return err
}

batch, err := conn.PrepareBatch(ctx, "INSERT INTO example")
if err != nil {
    return err
}

if err = batch.Append(
    orb.Point{11, 22},
    orb.Ring{
        orb.Point{1, 2},
        orb.Point{1, 2},
    },
    orb.Polygon{
        orb.Ring{
            orb.Point{1, 2},
            orb.Point{12, 2},
        },
        orb.Ring{
            orb.Point{11, 2},
            orb.Point{1, 12},
        },
    },
    orb.MultiPolygon{
        orb.Polygon{
            orb.Ring{
                orb.Point{1, 2},
                orb.Point{12, 2},
            },
            orb.Ring{
                orb.Point{11, 2},
                orb.Point{1, 12},
            },
        },
        orb.Polygon{
            orb.Ring{
                orb.Point{1, 2},
                orb.Point{12, 2},
            },
            orb.Ring{
                orb.Point{11, 2},
                orb.Point{1, 12},
            },
        },
    },
); err != nil {
    return err
}

if err = batch.Send(); err != nil {
    return err
}

var (
    point    orb.Point
    ring     orb.Ring
    polygon  orb.Polygon
    mPolygon orb.MultiPolygon
)

if err = conn.QueryRow(ctx, "SELECT * FROM example").Scan(&point, &ring, &polygon, &mPolygon); err != nil {
    return err
}

Full Example

UUID

The UUID type is supported by the github.com/google/uuid package. Users can also send and marshall uuids as strings or any type which implements sql.Scanner or Stringify.

if err = conn.Exec(ctx, `
    CREATE TABLE example (
            col1 UUID,
            col2 UUID
        )
        Engine Memory
    `); err != nil {
    return err
}

batch, err := conn.PrepareBatch(ctx, "INSERT INTO example")
if err != nil {
    return err
}
col1Data, _ := uuid.NewUUID()
if err = batch.Append(
    col1Data,
    "603966d6-ed93-11ec-8ea0-0242ac120002",
); err != nil {
    return err
}

if err = batch.Send(); err != nil {
    return err
}

var (
    col1 uuid.UUID
    col2 uuid.UUID
)

if err = conn.QueryRow(ctx, "SELECT * FROM example").Scan(&col1, &col2); err != nil {
    return err
}

Full Example

Decimal

The Decimal type is supported by github.com/shopspring/decimal package.

if err = conn.Exec(ctx, `
    CREATE TABLE example (
        Col1 Decimal32(3),
        Col2 Decimal(18,6),
        Col3 Decimal(15,7),
        Col4 Decimal128(8),
        Col5 Decimal256(9)
    ) Engine Memory
    `); err != nil {
    return err
}

batch, err := conn.PrepareBatch(ctx, "INSERT INTO example")
if err != nil {
    return err
}
if err = batch.Append(
    decimal.New(25, 4),
    decimal.New(30, 5),
    decimal.New(35, 6),
    decimal.New(135, 7),
    decimal.New(256, 8),
); err != nil {
    return err
}

if err = batch.Send(); err != nil {
    return err
}

var (
    col1 decimal.Decimal
    col2 decimal.Decimal
    col3 decimal.Decimal
    col4 decimal.Decimal
    col5 decimal.Decimal
)

if err = conn.QueryRow(ctx, "SELECT * FROM example").Scan(&col1, &col2, &col3, &col4, &col5); err != nil {
    return err
}
fmt.Printf("col1=%v, col2=%v, col3=%v, col4=%v, col5=%v\n", col1, col2, col3, col4, col5)

Full Example

Nullable

The go value of Nil represents a ClickHouse NULL. This can be used if a field is declared Nullable. At insert time, Nil can be passed for both the normal and Nullable version of a column. For the former, the default value for the type will be persisted, e.g., an empty string for string. For the nullable version, a NULL value will be stored in ClickHouse.

At Scan time, the user must pass a pointer to a type that supports nil, e.g., *string, in order to represent the nil value for a Nullable field. In the example below, col1, which is a Nullable(String), thus receives a **string. This allows nil to be represented.

if err = conn.Exec(ctx, `
    CREATE TABLE example (
            col1 Nullable(String),
            col2 String,
            col3 Nullable(Int8),
            col4 Nullable(Int64)
        )
        Engine Memory
    `); err != nil {
    return err
}

batch, err := conn.PrepareBatch(ctx, "INSERT INTO example")
if err != nil {
    return err
}
if err = batch.Append(
    nil,
    nil,
    nil,
    sql.NullInt64{Int64: 0, Valid: false},
); err != nil {
    return err
}

if err = batch.Send(); err != nil {
    return err
}

var (
    col1 *string
    col2 string
    col3 *int8
    col4 sql.NullInt64
)

if err = conn.QueryRow(ctx, "SELECT * FROM example").Scan(&col1, &col2, &col3, &col4); err != nil {
    return err
}

Full Example

The client additionally supports the sql.Null* types e.g. sql.NullInt64. These are compatible with their equivalent ClickHouse types.

Big Ints - Int128, Int256, UInt128, UInt256

Number types larger than 64 bits are represented using the native go big package.

if err = conn.Exec(ctx, `
    CREATE TABLE example (
        Col1 Int128,
        Col2 UInt128,
        Col3 Array(Int128),
        Col4 Int256,
        Col5 Array(Int256),
        Col6 UInt256,
        Col7 Array(UInt256)
    ) Engine Memory`); err != nil {
    return err
}

batch, err := conn.PrepareBatch(ctx, "INSERT INTO example")
if err != nil {
    return err
}

col1Data, _ := new(big.Int).SetString("170141183460469231731687303715884105727", 10)
col2Data := big.NewInt(128)
col3Data := []*big.Int{
    big.NewInt(-128),
    big.NewInt(128128),
    big.NewInt(128128128),
}
col4Data := big.NewInt(256)
col5Data := []*big.Int{
    big.NewInt(256),
    big.NewInt(256256),
    big.NewInt(256256256256),
}
col6Data := big.NewInt(256)
col7Data := []*big.Int{
    big.NewInt(256),
    big.NewInt(256256),
    big.NewInt(256256256256),
}

if err = batch.Append(col1Data, col2Data, col3Data, col4Data, col5Data, col6Data, col7Data); err != nil {
    return err
}

if err = batch.Send(); err != nil {
    return err
}

var (
    col1 big.Int
    col2 big.Int
    col3 []*big.Int
    col4 big.Int
    col5 []*big.Int
    col6 big.Int
    col7 []*big.Int
)

if err = conn.QueryRow(ctx, "SELECT * FROM example").Scan(&col1, &col2, &col3, &col4, &col5, &col6, &col7); err != nil {
    return err
}
fmt.Printf("col1=%v, col2=%v, col3=%v, col4=%v, col5=%v, col6=%v, col7=%v\n", col1, col2, col3, col4, col5, col6, col7)

Full Example

Compression

Support for compression methods depends on the underlying protocol in use. For the native protocol, the client supports LZ4 and ZSTD compression. This is performed at a block level only. Compression can be enabled by including a Compression configuration with the connection.

conn, err := clickhouse.Open(&clickhouse.Options{
    Addr: []string{fmt.Sprintf("%s:%d", env.Host, env.Port)},
    Auth: clickhouse.Auth{
        Database: env.Database,
        Username: env.Username,
        Password: env.Password,
    },
    Compression: &clickhouse.Compression{
        Method: clickhouse.CompressionZSTD,
    },
    MaxOpenConns: 1,
})
ctx := context.Background()
defer func() {
    conn.Exec(ctx, "DROP TABLE example")
}()
conn.Exec(context.Background(), "DROP TABLE IF EXISTS example")
if err = conn.Exec(ctx, `
    CREATE TABLE example (
            Col1 Array(String)
    ) Engine Memory
    `); err != nil {
    return err
}
batch, err := conn.PrepareBatch(ctx, "INSERT INTO example")
if err != nil {
    return err
}
for i := 0; i < 1000; i++ {
    if err := batch.Append([]string{strconv.Itoa(i), strconv.Itoa(i + 1), strconv.Itoa(i + 2), strconv.Itoa(i + 3)}); err != nil {
        return err
    }
}
if err := batch.Send(); err != nil {
    return err
}

Full Example

Additional compression techniques are available if using the standard interface over HTTP. See database/sql API - Compression for further details.

Parameter Binding

The client supports parameter binding for the Exec, Query, and QueryRow methods. As shown in the example below, this is supported using named, numbered, and positional parameters. We provide examples of these below.

var count uint64
// positional bind
if err = conn.QueryRow(ctx, "SELECT count() FROM example WHERE Col1 >= ? AND Col3 < ?", 500, now.Add(time.Duration(750)*time.Second)).Scan(&count); err != nil {
    return err
}
// 250
fmt.Printf("Positional bind count: %d\n", count)
// numeric bind
if err = conn.QueryRow(ctx, "SELECT count() FROM example WHERE Col1 <= $2 AND Col3 > $1", now.Add(time.Duration(150)*time.Second), 250).Scan(&count); err != nil {
    return err
}
// 100
fmt.Printf("Numeric bind count: %d\n", count)
// named bind
if err = conn.QueryRow(ctx, "SELECT count() FROM example WHERE Col1 <= @col1 AND Col3 > @col3", clickhouse.Named("col1", 100), clickhouse.Named("col3", now.Add(time.Duration(50)*time.Second))).Scan(&count); err != nil {
    return err
}
// 50
fmt.Printf("Named bind count: %d\n", count)

Full Example

Special Cases

By default, slices will be unfolded into a comma-separated list of values if passed as a parameter to a query. If users require a set of values to be injected with wrapping [ ], ArraySet should be used.

If groups/tuples are required, with wrapping ( ) e.g., for use with IN operators, users can use a GroupSet. This is particularly useful for cases where multiple groups are required, as shown in the example below.

Finally, DateTime64 fields require precision in order to ensure parameters are rendered appropriately. The precision level for the field is unknown by the client, however, so the user must provide it. To facilitate this, we provide the DateNamed parameter.

var count uint64
// arrays will be unfolded
if err = conn.QueryRow(ctx, "SELECT count() FROM example WHERE Col1 IN (?)", []int{100, 200, 300, 400, 500}).Scan(&count); err != nil {
    return err
}
fmt.Printf("Array unfolded count: %d\n", count)
// arrays will be preserved with []
if err = conn.QueryRow(ctx, "SELECT count() FROM example WHERE Col4 = ?", clickhouse.ArraySet{300, 301}).Scan(&count); err != nil {
    return err
}
fmt.Printf("Array count: %d\n", count)
// Group sets allow us to form ( ) lists
if err = conn.QueryRow(ctx, "SELECT count() FROM example WHERE Col1 IN ?", clickhouse.GroupSet{[]interface{}{100, 200, 300, 400, 500}}).Scan(&count); err != nil {
    return err
}
fmt.Printf("Group count: %d\n", count)
// More useful when we need nesting
if err = conn.QueryRow(ctx, "SELECT count() FROM example WHERE (Col1, Col5) IN (?)", []clickhouse.GroupSet{{[]interface{}{100, 101}}, {[]interface{}{200, 201}}}).Scan(&count); err != nil {
    return err
}
fmt.Printf("Group count: %d\n", count)
// Use DateNamed when you need a precision in your time#
if err = conn.QueryRow(ctx, "SELECT count() FROM example WHERE Col3 >= @col3", clickhouse.DateNamed("col3", now.Add(time.Duration(500)*time.Millisecond), clickhouse.NanoSeconds)).Scan(&count); err != nil {
    return err
}
fmt.Printf("NamedDate count: %d\n", count)

Full Example

Using Context

Go contexts provide a means of passing deadlines, cancellation signals, and other request-scoped values across API boundaries. All methods on a connection accept a context as their first variable. While previous examples used context.Background(), users can use this capability to pass settings and deadlines and to cancel queries.

Passing a context created withDeadline allows execution time limits to be placed on queries. Not this is an absolute time and expiry will only release the connection and send a cancel signal to ClickHouse. WithCancel can alternatively be used to cancel a query explicitly.

The helpers clickhouse.WithQueryID and clickhouse.WithQuotaKey allow a query id and quota key to be specified. Query ids can be useful for tracking queries in logs and for cancellation purposes. A quota key can be used to impose limits on ClickHouse usage based on a unique key value - see Quotas Management for further details.

Users can also use the context to ensure a setting is only applied for a specific query - rather than for the entire connection, as shown in Connection Settings.

Finally, users can control the size of the block buffer via the clickhouse.WithBlockSize. This overrides the connection level setting BlockBufferSize and controls the maximum number of blocks that are decoded and held in memory at any time. Larger values potentially mean more parallelization at the expense of memory.

Examples of the above are shown below.

dialCount := 0
conn, err := clickhouse.Open(&clickhouse.Options{
    Addr: []string{fmt.Sprintf("%s:%d", env.Host, env.Port)},
    Auth: clickhouse.Auth{
        Database: env.Database,
        Username: env.Username,
        Password: env.Password,
    },
    DialContext: func(ctx context.Context, addr string) (net.Conn, error) {
        dialCount++
        var d net.Dialer
        return d.DialContext(ctx, "tcp", addr)
    },
})
if err != nil {
    return err
}
if err := clickhouse_tests.CheckMinServerServerVersion(conn, 22, 6, 1); err != nil {
    return nil
}
// we can use context to pass settings to a specific API call
ctx := clickhouse.Context(context.Background(), clickhouse.WithSettings(clickhouse.Settings{
    "allow_experimental_object_type": "1",
}))

conn.Exec(ctx, "DROP TABLE IF EXISTS example")

// to create a JSON column we need allow_experimental_object_type=1
if err = conn.Exec(ctx, `
    CREATE TABLE example (
            Col1 JSON
        )
        Engine Memory
    `); err != nil {
    return err
}

// queries can be cancelled using the context
ctx, cancel := context.WithCancel(context.Background())
go func() {
    cancel()
}()
if err = conn.QueryRow(ctx, "SELECT sleep(3)").Scan(); err == nil {
    return fmt.Errorf("expected cancel")
}

// set a deadline for a query - this will cancel the query after the absolute time is reached.
// queries will continue to completion in ClickHouse
ctx, cancel = context.WithDeadline(context.Background(), time.Now().Add(-time.Second))
defer cancel()
if err := conn.Ping(ctx); err == nil {
    return fmt.Errorf("expected deadline exceeeded")
}

// set a query id to assist tracing queries in logs e.g. see system.query_log
var one uint8
queryId, _ := uuid.NewUUID()
ctx = clickhouse.Context(context.Background(), clickhouse.WithQueryID(queryId.String()))
if err = conn.QueryRow(ctx, "SELECT 1").Scan(&one); err != nil {
    return err
}

conn.Exec(context.Background(), "DROP QUOTA IF EXISTS foobar")
defer func() {
    conn.Exec(context.Background(), "DROP QUOTA IF EXISTS foobar")
}()
ctx = clickhouse.Context(context.Background(), clickhouse.WithQuotaKey("abcde"))
// set a quota key - first create the quota
if err = conn.Exec(ctx, "CREATE QUOTA IF NOT EXISTS foobar KEYED BY client_key FOR INTERVAL 1 minute MAX queries = 5 TO default"); err != nil {
    return err
}

type Number struct {
    Number uint64 `ch:"number"`
}
for i := 1; i <= 6; i++ {
    var result []Number
    if err = conn.Select(ctx, &result, "SELECT number FROM numbers(10)"); err != nil {
        return err
    }
}

Full Example

Progress/Profile/Log Information

Progress, Profile, and Log information can be requested on queries. Progress information will report statistics on the number of rows and bytes that have been read and processed in ClickHouse. Conversely, Profile information provides a summary of data returned to the client, including totals of bytes, rows, and blocks. Finally, log information provides statistics on threads, e.g., memory usage and data speed.

Obtaining this information requires the user to use Context, to which the user can pass call-back functions.

totalRows := uint64(0)
// use context to pass a call back for progress and profile info
ctx := clickhouse.Context(context.Background(), clickhouse.WithProgress(func(p *clickhouse.Progress) {
    fmt.Println("progress: ", p)
    totalRows += p.Rows
}), clickhouse.WithProfileInfo(func(p *clickhouse.ProfileInfo) {
    fmt.Println("profile info: ", p)
}), clickhouse.WithLogs(func(log *clickhouse.Log) {
    fmt.Println("log info: ", log)
}))

rows, err := conn.Query(ctx, "SELECT number from numbers(1000000) LIMIT 1000000")
if err != nil {
    return err
}
for rows.Next() {
}

fmt.Printf("Total Rows: %d\n", totalRows)
rows.Close()

Full Example

Dynamic Scanning

Users may need to read tables for which they do not know the schema or type of the fields being returned. This is common in cases where ad-hoc data analysis is performed or generic tooling is written. To achieve this, column-type information is available on query responses. This can be used with Go reflection to create runtime instances of correctly typed variables which can be passed to Scan.

const query = `
SELECT
        1     AS Col1
    , 'Text' AS Col2
`
rows, err := conn.Query(context.Background(), query)
if err != nil {
    return err
}
var (
    columnTypes = rows.ColumnTypes()
    vars        = make([]interface{}, len(columnTypes))
)
for i := range columnTypes {
    vars[i] = reflect.New(columnTypes[i].ScanType()).Interface()
}
for rows.Next() {
    if err := rows.Scan(vars...); err != nil {
        return err
    }
    for _, v := range vars {
        switch v := v.(type) {
        case *string:
            fmt.Println(*v)
        case *uint8:
            fmt.Println(*v)
        }
    }
}

Full Example

External tables

External tables allow the client to send data to ClickHouse, with a SELECT query. This data is put in a temporary table and can be used in the query itself for evaluation.

To send external data to the client with a query, the user must build an external table via ext.NewTable before passing this via the context.

table1, err := ext.NewTable("external_table_1",
    ext.Column("col1", "UInt8"),
    ext.Column("col2", "String"),
    ext.Column("col3", "DateTime"),
)
if err != nil {
    return err
}

for i := 0; i < 10; i++ {
    if err = table1.Append(uint8(i), fmt.Sprintf("value_%d", i), time.Now()); err != nil {
        return err
    }
}

table2, err := ext.NewTable("external_table_2",
    ext.Column("col1", "UInt8"),
    ext.Column("col2", "String"),
    ext.Column("col3", "DateTime"),
)

for i := 0; i < 10; i++ {
    table2.Append(uint8(i), fmt.Sprintf("value_%d", i), time.Now())
}
ctx := clickhouse.Context(context.Background(),
    clickhouse.WithExternalTable(table1, table2),
)
rows, err := conn.Query(ctx, "SELECT * FROM external_table_1")
if err != nil {
    return err
}
for rows.Next() {
    var (
        col1 uint8
        col2 string
        col3 time.Time
    )
    rows.Scan(&col1, &col2, &col3)
    fmt.Printf("col1=%d, col2=%s, col3=%v\n", col1, col2, col3)
}
rows.Close()

var count uint64
if err := conn.QueryRow(ctx, "SELECT COUNT(*) FROM external_table_1").Scan(&count); err != nil {
    return err
}
fmt.Printf("external_table_1: %d\n", count)
if err := conn.QueryRow(ctx, "SELECT COUNT(*) FROM external_table_2").Scan(&count); err != nil {
    return err
}
fmt.Printf("external_table_2: %d\n", count)
if err := conn.QueryRow(ctx, "SELECT COUNT(*) FROM (SELECT * FROM external_table_1 UNION ALL SELECT * FROM external_table_2)").Scan(&count); err != nil {
    return err
}
fmt.Printf("external_table_1 UNION external_table_2: %d\n", count)

Full Example

Open Telemetry

ClickHouse allows a trace context to be passed as part of the native protocol. The client allows a Span to be created via the function clickhouse.withSpan and passed via the Context to achieve this.

var count uint64
rows := conn.QueryRow(clickhouse.Context(context.Background(), clickhouse.WithSpan(
    trace.NewSpanContext(trace.SpanContextConfig{
        SpanID:  trace.SpanID{1, 2, 3, 4, 5},
        TraceID: trace.TraceID{5, 4, 3, 2, 1},
    }),
)), "SELECT COUNT() FROM (SELECT number FROM system.numbers LIMIT 5)")
if err := rows.Scan(&count); err != nil {
    return err
}
fmt.Printf("count: %d\n", count)

Full Example

Full details on exploiting tracing can be found under OpenTelemetry support.

Database/SQL API

The database/sql or “standard” API allows users to use the client in scenarios where application code should be agnostic of the underlying databases by conforming to a standard interface. This comes at some expense - additional layers of abstraction and indirection and primitives which are not necessarily aligned with ClickHouse. These costs are, however, typically acceptable in scenarios where tooling needs to connect to multiple databases.

Additionally, this client supports using HTTP as the transport layer - data will still be encoded in the native format for optimal performance.

The following aims to mirror the structure of the documentation for the ClickHouse API.

Full code examples for the standard API can be found here.

Connecting

Connection can be achieved either via a DSN string with the format clickhouse://<host>:<port>?<query_option>=<value> and Open method or via the clickhouse.OpenDB method. The latter is not part of the database/sql specification but returns a sql.DB instance. This method provides functionality such as profiling, for which there are no obvious means of exposing through the database/sql specification.

func Connect() error {
    env, err := GetStdTestEnvironment()
    if err != nil {
        return err
    }
    conn := clickhouse.OpenDB(&clickhouse.Options{
        Addr: []string{fmt.Sprintf("%s:%d", env.Host, env.Port)},
        Auth: clickhouse.Auth{
            Database: env.Database,
            Username: env.Username,
            Password: env.Password,
        },
    })
    return conn.Ping()
}


func ConnectDSN() error {
    env, err := GetStdTestEnvironment()
    if err != nil {
        return err
    }
    conn, err := sql.Open("clickhouse", fmt.Sprintf("clickhouse://%s:%d?username=%s&password=%s", env.Host, env.Port, env.Username, env.Password))
    if err != nil {
        return err
    }
    return conn.Ping()
}

Full Example

For all subsequent examples, unless explicitly shown, we assume the use of the ClickHouse conn variable has been created and is available.

Connection Settings

The following parameters can be passed in the DSN string:

• hosts - comma-separated list of single address hosts for load-balancing and failover - see Connecting to Multiple Nodes.
• username/password - auth credentials - see Authentication
• database - select the current default database
• dial_timeout - a duration string is a possibly signed sequence of decimal numbers, each with optional fraction and a unit suffix such as 300ms, 1s. Valid time units are ms, s, m.
• connection_open_strategy - random/in_order (default random) - see Connecting to Multiple Nodes
  • round_robin - choose a round-robin server from the set
  • in_order - first live server is chosen in specified order
• debug - enable debug output (boolean value)
• compress - specify the compression algorithm - none (default), zstd, lz4, gzip, deflate, br. If set to true, lz4 will be used. Only lz4 and zstd are supported for native communication.
• compress_level - Level of compression (default is 0). See Compression. This is algorithm specific:
  • gzip - -2 (Best Speed) to 9 (Best Compression)
  • deflate - -2 (Best Speed) to 9 (Best Compression)
  • br - 0 (Best Speed) to 11 (Best Compression)
  • zstd, lz4 - ignored
• secure - establish secure SSL connection (default is false)
• skip_verify - skip certificate verification (default is false)
• block_buffer_size - allows users to control the block buffer size. See BlockBufferSize. (default is 2)

func ConnectSettings() error {
    env, err := GetStdTestEnvironment()
    if err != nil {
        return err
    }
    conn, err := sql.Open("clickhouse", fmt.Sprintf("clickhouse://127.0.0.1:9001,127.0.0.1:9002,%s:%d/%s?username=%s&password=%s&dial_timeout=10s&connection_open_strategy=round_robin&debug=true&compress=lz4", env.Host, env.Port, env.Database, env.Username, env.Password))
    if err != nil {
        return err
    }
    return conn.Ping()
}

Full Example

Connection Pooling

Users can influence the use of the provided list of node addresses as described in Connecting to Multiple Nodes. Connection management and pooling is, however, delegated to sql.DB by design.

Connecting over HTTP

By default, connections are established over the native protocol. For users needing HTTP, this can be enabled by either modifying the DSN to include the HTTP protocol or by specifying the Protocol in the connection options.

func ConnectHTTP() error {
    env, err := GetStdTestEnvironment()
    if err != nil {
        return err
    }
    conn := clickhouse.OpenDB(&clickhouse.Options{
        Addr: []string{fmt.Sprintf("%s:%d", env.Host, env.HttpPort)},
        Auth: clickhouse.Auth{
            Database: env.Database,
            Username: env.Username,
            Password: env.Password,
        },
        Protocol: clickhouse.HTTP,
    })
    return conn.Ping()
}

func ConnectDSNHTTP() error {
    env, err := GetStdTestEnvironment()
    if err != nil {
        return err
    }
    conn, err := sql.Open("clickhouse", fmt.Sprintf("http://%s:%d?username=%s&password=%s", env.Host, env.HttpPort, env.Username, env.Password))
    if err != nil {
        return err
    }
    return conn.Ping()
}

Full Example

Connecting to Multiple Nodes

If using OpenDB, connect to multiple hosts using the same options approach as that used for the ClickHouse API - optionally specifying the ConnOpenStrategy.

For DSN-based connections, the string accepts multiple hosts and a connection_open_strategy parameter for which the value round_robin or in_order can be set.

func MultiStdHost() error {
    env, err := GetStdTestEnvironment()
    if err != nil {
        return err
    }
    conn, err := clickhouse.Open(&clickhouse.Options{
        Addr: []string{"127.0.0.1:9001", "127.0.0.1:9002", fmt.Sprintf("%s:%d", env.Host, env.Port)},
        Auth: clickhouse.Auth{
            Database: env.Database,
            Username: env.Username,
            Password: env.Password,
        },
        ConnOpenStrategy: clickhouse.ConnOpenRoundRobin,
    })
    if err != nil {
        return err
    }
    v, err := conn.ServerVersion()
    if err != nil {
        return err
    }
    fmt.Println(v.String())
    return nil
}

func MultiStdHostDSN() error {
    env, err := GetStdTestEnvironment()
    if err != nil {
        return err
    }
    conn, err := sql.Open("clickhouse", fmt.Sprintf("clickhouse://127.0.0.1:9001,127.0.0.1:9002,%s:%d?username=%s&password=%s&connection_open_strategy=round_robin", env.Host, env.Port, env.Username, env.Password))
    if err != nil {
        return err
    }
    return conn.Ping()
}

Full Example

Using TLS

If using a DSN connection string, SSL can be enabled via the parameter “secure=true”. The OpenDB method utilizes the same approach as the native API for TLS, relying on the specification of a non-nil TLS struct. While the DSN connection string supports the parameter skip_verify to skip SSL verification, the OpenDB method is required for more advanced TLS configurations - since it permits the passing of a configuration.

func ConnectSSL() error {
    env, err := GetStdTestEnvironment()
    if err != nil {
        return err
    }
    cwd, err := os.Getwd()
    if err != nil {
        return err
    }
    t := &tls.Config{}
    caCert, err := ioutil.ReadFile(path.Join(cwd, "../../tests/resources/CAroot.crt"))
    if err != nil {
        return err
    }
    caCertPool := x509.NewCertPool()
    successful := caCertPool.AppendCertsFromPEM(caCert)
    if !successful {
        return err
    }
    t.RootCAs = caCertPool


    conn := clickhouse.OpenDB(&clickhouse.Options{
        Addr: []string{fmt.Sprintf("%s:%d", env.Host, env.SslPort)},
        Auth: clickhouse.Auth{
            Database: env.Database,
            Username: env.Username,
            Password: env.Password,
        },
        TLS: t,
    })
    return conn.Ping()
}

func ConnectDSNSSL() error {
    env, err := GetStdTestEnvironment()
    if err != nil {
        return err
    }
    conn, err := sql.Open("clickhouse", fmt.Sprintf("https://%s:%d?secure=true&skip_verify=true&username=%s&password=%s", env.Host, env.HttpsPort, env.Username, env.Password))
    if err != nil {
        return err
    }
    return conn.Ping()
}

Full Example

Authentication

If using OpenDB, authentication information can be passed via the usual options. For DSN-based connections, a username and password can be passed in the connection string - either as parameters or as credentials encoded in the address.

func ConnectAuth() error {
    env, err := GetStdTestEnvironment()
    if err != nil {
        return err
    }
    conn := clickhouse.OpenDB(&clickhouse.Options{
        Addr: []string{fmt.Sprintf("%s:%d", env.Host, env.Port)},
        Auth: clickhouse.Auth{
            Database: env.Database,
            Username: env.Username,
            Password: env.Password,
        },
    })
    return conn.Ping()
}

func ConnectDSNAuth() error {
    env, err := GetStdTestEnvironment()
    conn, err := sql.Open("clickhouse", fmt.Sprintf("http://%s:%d?username=%s&password=%s", env.Host, env.HttpPort, env.Username, env.Password))
    if err != nil {
        return err
    }
    if err = conn.Ping(); err != nil {
        return err
    }
    conn, err = sql.Open("clickhouse", fmt.Sprintf("http://%s:%s@%s:%d", env.Username, env.Password, env.Host, env.HttpPort))
    if err != nil {
        return err
    }
    return conn.Ping()
}

Full Example

Execution

Once a connection has been obtained, users can issue sql statements for execution via the Exec method.

conn.Exec(`DROP TABLE IF EXISTS example`)
_, err = conn.Exec(`
    CREATE TABLE IF NOT EXISTS example (
        Col1 UInt8,
        Col2 String
    ) engine=Memory
`)
if err != nil {
    return err
}
_, err = conn.Exec("INSERT INTO example VALUES (1, 'test-1')")

Full Example

This method does not support receiving a context - by default, it executes with the background context. Users can use ExecContext if this is needed - see Using Context.

Batch Insert

Batch semantics can be achieved by creating a sql.Tx via the Being method. From this, a batch can be obtained using the Prepare method with the INSERT statement. This returns a sql.Stmt to which rows can be appended using the Exec method. The batch will be accumulated in memory until Commit is executed on the original sql.Tx.

batch, err := scope.Prepare("INSERT INTO example")
if err != nil {
    return err
}
for i := 0; i < 1000; i++ {
    _, err := batch.Exec(
        uint8(42),
        "ClickHouse", "Inc",
        uuid.New(),
        map[string]uint8{"key": 1},             // Map(String, UInt8)
        []string{"Q", "W", "E", "R", "T", "Y"}, // Array(String)
        []interface{}{ // Tuple(String, UInt8, Array(Map(String, String)))
            "String Value", uint8(5), []map[string]string{
                map[string]string{"key": "value"},
                map[string]string{"key": "value"},
                map[string]string{"key": "value"},
            },
        },
        time.Now(),
    )
    if err != nil {
        return err
    }
}
return scope.Commit()

Full Example

Querying Row/s

Querying a single row can be achieved using the QueryRow method. This returns a *sql.Row, on which Scan can be invoked with pointers to variables into which the columns should be marshaled. A QueryRowContext variant allows a context to be passed other than background - see Using Context.

row := conn.QueryRow("SELECT * FROM example")
var (
    col1             uint8
    col2, col3, col4 string
    col5             map[string]uint8
    col6             []string
    col7             interface{}
    col8             time.Time
)
if err := row.Scan(&col1, &col2, &col3, &col4, &col5, &col6, &col7, &col8); err != nil {
    return err
}

Full Example

Iterating multiple rows requires the Query method. This returns a *sql.Rows struct on which Next can be invoked to iterate through the rows. QueryContext equivalent allows passing of a context.

rows, err := conn.Query("SELECT * FROM example")
if err != nil {
    return err
}
var (
    col1             uint8
    col2, col3, col4 string
    col5             map[string]uint8
    col6             []string
    col7             interface{}
    col8             time.Time
)
for rows.Next() {
    if err := rows.Scan(&col1, &col2, &col3, &col4, &col5, &col6, &col7, &col8); err != nil {
        return err
    }
    fmt.Printf("row: col1=%d, col2=%s, col3=%s, col4=%s, col5=%v, col6=%v, col7=%v, col8=%v\n", col1, col2, col3, col4, col5, col6, col7, col8)
}

Full Example

Async Insert

Asynchronous inserts can be achieved by executing an insert via the ExecContext method. This should be passed a context with asynchronous mode enabled, as shown below. This allows the user to specify whether the client should wait for the server to complete the insert or respond once the data has been received. This effectively controls the parameter wait_for_async_insert.

const ddl = `
    CREATE TABLE example (
            Col1 UInt64
        , Col2 String
        , Col3 Array(UInt8)
        , Col4 DateTime
    ) ENGINE = Memory
    `
if _, err := conn.Exec(ddl); err != nil {
    return err
}
ctx := clickhouse.Context(context.Background(), clickhouse.WithStdAsync(false))
{
    for i := 0; i < 100; i++ {
        _, err := conn.ExecContext(ctx, fmt.Sprintf(`INSERT INTO example VALUES (
            %d, '%s', [1, 2, 3, 4, 5, 6, 7, 8, 9], now()
        )`, i, "Golang SQL database driver"))
        if err != nil {
            return err
        }
    }
}

Full Example

Columnar Insert

Not supported using the standard interface.

Using Structs

Not supported using the standard interface.

Type Conversions

The standard database/sql interface should support the same types as the ClickHouse API. There are a few exceptions, primarily for complex types, that we document below. Similar to the ClickHouse API, the client aims to be as flexible as possible concerning accepting variable types for both insertion and marshaling of responses. See Type Conversions for further details.

Complex Types

Unless stated, complex type handling should be the same as the ClickHouse API. Differences are a result of database/sql internals.

Maps

Unlike the ClickHouse API, the standard API requires maps to be strongly typed at scan type. For example, users cannot pass a map[string]interface{} for a Map(String,String) field and must use a map[string]string instead. An interface{} variable will always be compatible and can be used for more complex structures. Structs are not supported at read time.

var (
    col1Data = map[string]uint64{
        "key_col_1_1": 1,
        "key_col_1_2": 2,
    }
    col2Data = map[string]uint64{
        "key_col_2_1": 10,
        "key_col_2_2": 20,
    }
    col3Data = map[string]uint64{}
    col4Data = []map[string]string{
        {"A": "B"},
        {"C": "D"},
    }
    col5Data = map[string]uint64{
        "key_col_5_1": 100,
        "key_col_5_2": 200,
    }
)
if _, err := batch.Exec(col1Data, col2Data, col3Data, col4Data, col5Data); err != nil {
    return err
}
if err = scope.Commit(); err != nil {
    return err
}
var (
    col1 interface{}
    col2 map[string]uint64
    col3 map[string]uint64
    col4 []map[string]string
    col5 map[string]uint64
)
if err := conn.QueryRow("SELECT * FROM example").Scan(&col1, &col2, &col3, &col4, &col5); err != nil {
    return err
}
fmt.Printf("col1=%v, col2=%v, col3=%v, col4=%v, col5=%v", col1, col2, col3, col4, col5)

Full Example

Insert behavior is the same as the ClickHouse API.

JSON

The standard API does not support structs or maps at read time. Users must pass an interface{} variable to the Scan method.

rows = conn.QueryRow("SELECT event.assignee.Achievement FROM example")
var achievement interface{}
if err = rows.Scan(&achievement); err != nil {
    return err
}
fmt.Println(clickhouse_tests.ToJson(event))
rows = conn.QueryRow("SELECT event.assignee.Repositories FROM example")
var repositories interface{}
if err = rows.Scan(&repositories); err != nil {
    return err
}

Full Example

Insert behavior is the same as the ClickHouse API.

Compression

The standard API supports the same compression algorithms as native ClickHouse API i.e. lz4 and zstd compression at a block level. In addition, gzip, deflate and br compression are supported for HTTP connections. If any of these are enabled, compression is performed on blocks during insertion and for query responses. Other requests e.g. pings or query requests, will remain uncompressed. This is consistent with lz4 and zstd options.

If using the OpenDB method to establish a connection, a Compression configuration can be passed. This includes the ability to specify the compression level (see below). If connecting via sql.Open with DSN, utilize the parameter compress. This can either be a specific compression algorithm i.e. gzip, deflate, br, zstd or lz4 or a boolean flag. If set to true, lz4 will be used. The default is none i.e. compression disabled.

conn := clickhouse.OpenDB(&clickhouse.Options{
    Addr: []string{fmt.Sprintf("%s:%d", env.Host, env.HttpPort)},
    Auth: clickhouse.Auth{
        Database: env.Database,
        Username: env.Username,
        Password: env.Password,
    },
    Compression: &clickhouse.Compression{
        Method: clickhouse.CompressionBrotli,
        Level:  5,
    },
    Protocol: clickhouse.HTTP,
})

Full Example

conn, err := sql.Open("clickhouse", fmt.Sprintf("http://%s:%d?username=%s&password=%s&compress=gzip&compress_level=5", env.Host, env.HttpPort, env.Username, env.Password))

Full Example

The level of applied compression can be controlled by the DSN parameter compress_level or the Level field of the Compression option. This defaults to 0 but is algorithm specific:

• gzip - -2 (Best Speed) to 9 (Best Compression)
• deflate - -2 (Best Speed) to 9 (Best Compression)
• br - 0 (Best Speed) to 11 (Best Compression)
• zstd, lz4 - ignored

Parameter Binding

The standard API supports the same parameter binding capabilities as the ClickHouse API, allowing parameters to be passed to the Exec, Query and QueryRow methods (and their equivalent Context variants). Positional, named and numbered parameters are supported.

var count uint64
// positional bind
if err = conn.QueryRow(ctx, "SELECT count() FROM example WHERE Col1 >= ? AND Col3 < ?", 500, now.Add(time.Duration(750)*time.Second)).Scan(&count); err != nil {
    return err
}
// 250
fmt.Printf("Positional bind count: %d\n", count)
// numeric bind
if err = conn.QueryRow(ctx, "SELECT count() FROM example WHERE Col1 <= $2 AND Col3 > $1", now.Add(time.Duration(150)*time.Second), 250).Scan(&count); err != nil {
    return err
}
// 100
fmt.Printf("Numeric bind count: %d\n", count)
// named bind
if err = conn.QueryRow(ctx, "SELECT count() FROM example WHERE Col1 <= @col1 AND Col3 > @col3", clickhouse.Named("col1", 100), clickhouse.Named("col3", now.Add(time.Duration(50)*time.Second))).Scan(&count); err != nil {
    return err
}
// 50
fmt.Printf("Named bind count: %d\n", count)

Full Example

Note special cases still apply.

Using Context

The standard API supports the same ability to pass deadlines, cancellation signals, and other request-scoped values via the context as the ClickHouse API. Unlike the ClickHouse API, this is achieved by using Context variants of the methods i.e. methods such as Exec, which use the background context by default, have a variant ExecContext to which a context can be passed as the first parameter. This allows a context to be passed at any stage of an application flow. For example, users can pass a context when establishing a connection via ConnContext or when requesting a query row via QueryRowContext. Examples of all available methods are shown below.

For more detail on using the context to pass deadlines, cancellation signals, query ids, quota keys and connection settings see Using Context for the ClickHouse API.

ctx := clickhouse.Context(context.Background(), clickhouse.WithSettings(clickhouse.Settings{
    "allow_experimental_object_type": "1",
}))
conn.ExecContext(ctx, "DROP TABLE IF EXISTS example")
// to create a JSON column we need allow_experimental_object_type=1
if _, err = conn.ExecContext(ctx, `
    CREATE TABLE example (
            Col1 JSON
        )
        Engine Memory
    `); err != nil {
    return err
}

// queries can be cancelled using the context
ctx, cancel := context.WithCancel(context.Background())
go func() {
    cancel()
}()
if err = conn.QueryRowContext(ctx, "SELECT sleep(3)").Scan(); err == nil {
    return fmt.Errorf("expected cancel")
}

// set a deadline for a query - this will cancel the query after the absolute time is reached. Again terminates the connection only,
// queries will continue to completion in ClickHouse
ctx, cancel = context.WithDeadline(context.Background(), time.Now().Add(-time.Second))
defer cancel()
if err := conn.PingContext(ctx); err == nil {
    return fmt.Errorf("expected deadline exceeeded")
}

// set a query id to assist tracing queries in logs e.g. see system.query_log
var one uint8
ctx = clickhouse.Context(context.Background(), clickhouse.WithQueryID(uuid.NewString()))
if err = conn.QueryRowContext(ctx, "SELECT 1").Scan(&one); err != nil {
    return err
}

conn.ExecContext(context.Background(), "DROP QUOTA IF EXISTS foobar")
defer func() {
    conn.ExecContext(context.Background(), "DROP QUOTA IF EXISTS foobar")
}()
ctx = clickhouse.Context(context.Background(), clickhouse.WithQuotaKey("abcde"))
// set a quota key - first create the quota
if _, err = conn.ExecContext(ctx, "CREATE QUOTA IF NOT EXISTS foobar KEYED BY client_key FOR INTERVAL 1 minute MAX queries = 5 TO default"); err != nil {
    return err
}

// queries can be cancelled using the context
ctx, cancel = context.WithCancel(context.Background())
// we will get some results before cancel
ctx = clickhouse.Context(ctx, clickhouse.WithSettings(clickhouse.Settings{
    "max_block_size": "1",
}))
rows, err := conn.QueryContext(ctx, "SELECT sleepEachRow(1), number FROM numbers(100);")
if err != nil {
    return err
}
var (
    col1 uint8
    col2 uint8
)

for rows.Next() {
    if err := rows.Scan(&col1, &col2); err != nil {
        if col2 > 3 {
            fmt.Println("expected cancel")
            return nil
        }
        return err
    }
    fmt.Printf("row: col2=%d\n", col2)
    if col2 == 3 {
        cancel()
    }
}

Full Example

Sessions

While native connections inherently have a session, connections over HTTP require the user to create a session id for passing in a context as a setting. This allows the use of features, e.g., Temporary tables, which are bound to a session.

conn := clickhouse.OpenDB(&clickhouse.Options{
    Addr: []string{fmt.Sprintf("%s:%d", env.Host, env.HttpPort)},
    Auth: clickhouse.Auth{
        Database: env.Database,
        Username: env.Username,
        Password: env.Password,
    },
    Protocol: clickhouse.HTTP,
    Settings: clickhouse.Settings{
        "session_id": uuid.NewString(),
    },
})
if _, err := conn.Exec(`DROP TABLE IF EXISTS example`); err != nil {
    return err
}
_, err = conn.Exec(`
    CREATE TEMPORARY TABLE IF NOT EXISTS example (
            Col1 UInt8
    )
`)
if err != nil {
    return err
}
scope, err := conn.Begin()
if err != nil {
    return err
}
batch, err := scope.Prepare("INSERT INTO example")
if err != nil {
    return err
}
for i := 0; i < 10; i++ {
    _, err := batch.Exec(
        uint8(i),
    )
    if err != nil {
        return err
    }
}
rows, err := conn.Query("SELECT * FROM example")
if err != nil {
    return err
}
var (
    col1 uint8
)
for rows.Next() {
    if err := rows.Scan(&col1); err != nil {
        return err
    }
    fmt.Printf("row: col1=%d\n", col1)
}

Full Example

Dynamic Scanning

Similar to the ClickHouse API, column type information is available to allow users to create runtime instances of correctly typed variables which can be passed to Scan. This allows columns to be read where the type is not known.

const query = `
SELECT
        1     AS Col1
    , 'Text' AS Col2
`
rows, err := conn.QueryContext(context.Background(), query)
if err != nil {
    return err
}
columnTypes, err := rows.ColumnTypes()
if err != nil {
    return err
}
vars := make([]interface{}, len(columnTypes))
for i := range columnTypes {
    vars[i] = reflect.New(columnTypes[i].ScanType()).Interface()
}
for rows.Next() {
    if err := rows.Scan(vars...); err != nil {
        return err
    }
    for _, v := range vars {
        switch v := v.(type) {
        case *string:
            fmt.Println(*v)
        case *uint8:
            fmt.Println(*v)
        }
    }
}

Full Example

External Tables

External tables allow the client to send data to ClickHouse, with a SELECT query. This data is put in a temporary table and can be used in the query itself for evaluation.

To send external data to the client with a query, the user must build an external table via ext.NewTable before passing this via the context.

table1, err := ext.NewTable("external_table_1",
    ext.Column("col1", "UInt8"),
    ext.Column("col2", "String"),
    ext.Column("col3", "DateTime"),
)
if err != nil {
    return err
}

for i := 0; i < 10; i++ {
    if err = table1.Append(uint8(i), fmt.Sprintf("value_%d", i), time.Now()); err != nil {
        return err
    }
}

table2, err := ext.NewTable("external_table_2",
    ext.Column("col1", "UInt8"),
    ext.Column("col2", "String"),
    ext.Column("col3", "DateTime"),
)

for i := 0; i < 10; i++ {
    table2.Append(uint8(i), fmt.Sprintf("value_%d", i), time.Now())
}
ctx := clickhouse.Context(context.Background(),
    clickhouse.WithExternalTable(table1, table2),
)
rows, err := conn.QueryContext(ctx, "SELECT * FROM external_table_1")
if err != nil {
    return err
}
for rows.Next() {
    var (
        col1 uint8
        col2 string
        col3 time.Time
    )
    rows.Scan(&col1, &col2, &col3)
    fmt.Printf("col1=%d, col2=%s, col3=%v\n", col1, col2, col3)
}
rows.Close()


var count uint64
if err := conn.QueryRowContext(ctx, "SELECT COUNT(*) FROM external_table_1").Scan(&count); err != nil {
    return err
}
fmt.Printf("external_table_1: %d\n", count)
if err := conn.QueryRowContext(ctx, "SELECT COUNT(*) FROM external_table_2").Scan(&count); err != nil {
    return err
}
fmt.Printf("external_table_2: %d\n", count)
if err := conn.QueryRowContext(ctx, "SELECT COUNT(*) FROM (SELECT * FROM external_table_1 UNION ALL SELECT * FROM external_table_2)").Scan(&count); err != nil {
    return err
}
fmt.Printf("external_table_1 UNION external_table_2: %d\n", count)

Full Example

Open Telemetry

ClickHouse allows a trace context to be passed as part of the native protocol. The client allows a Span to be created via the function clickhouse.withSpan and passed via the Context to achieve this. This is not supported when HTTP is used as transport.

var count uint64
rows := conn.QueryRowContext(clickhouse.Context(context.Background(), clickhouse.WithSpan(
    trace.NewSpanContext(trace.SpanContextConfig{
        SpanID:  trace.SpanID{1, 2, 3, 4, 5},
        TraceID: trace.TraceID{5, 4, 3, 2, 1},
    }),
)), "SELECT COUNT() FROM (SELECT number FROM system.numbers LIMIT 5)")
if err := rows.Scan(&count); err != nil {
    return err
}
fmt.Printf("count: %d\n", count)

Full Example

Performance Tips

• Utilize the ClickHouse API where possible, especially for primitive types. This avoids significant reflection and indirection.
• If reading large datasets, consider modifying the BlockBufferSize. This will increase the memory footprint but will mean more blocks can be decoded in parallel during row iteration. The default value of 2 is conservative and minimizes memory overhead. Higher values will mean more blocks in memory. This requires testing since different queries can produce different block sizes. It can therefore be set on a query level via the Context.
• Be specific with your types when inserting data. While the client aims to be flexible, e.g., allowing strings to be parsed for UUIDs or IPs, this requires data validation and incurs a cost at insert time.
• Use column-oriented inserts where possible. Again these should be strongly typed, avoiding the need for the client to convert your values.
• If using the JSON type, encoding of structs and maps to a columnar format is done on the client. This requires reflection, which can be expensive and more work for the client. Conversely, it requires less computation by ClickHouse at insert time. To shift computation to ClickHouse, insert data as a string. Where this work is performed is a design decision but ClickHouse will be more performant if you have cluster resource capacity.
• Follow ClickHouse recommendations for optimal insert performance.