Go-client configuration reference - ClickHouse Documentation

This page documents every configurable option in clickhouse-go v2.x. For a guide with code examples, see Configuration.

Version annotationsOptions added in clickhouse-go v2.35.0 or later are marked with (Since vX.Y.Z) next to their description. Options without a “Since” tag have been available since v2.0 and are present in every supported release.

How options are set

Options exist at three scopes:

Scope	How to set	Lifetime
Connection	`clickhouse.Options` struct or DSN string	All queries on the connection
Query	`clickhouse.Context()` with `WithXxx` functions	Single query execution
Batch	`PrepareBatch()` option functions	Single batch operation

Where scopes overlap, the more specific scope wins: Batch > Query > Connection. For Settings, query-level keys are merged with connection-level keys and query-level wins on conflict. Via Options struct:

conn, err := clickhouse.Open(&clickhouse.Options{
    Addr:        []string{"localhost:9000"},
    Auth:        clickhouse.Auth{Database: "default", Username: "default", Password: ""},
    DialTimeout: 10 * time.Second,
    Compression: &clickhouse.Compression{Method: clickhouse.CompressionLZ4},
})

Via DSN string:

db, err := sql.Open("clickhouse", "clickhouse://user:pass@localhost:9000/default?dial_timeout=10s&compress=lz4")

Via Connector (database/sql with Options struct):

db := sql.OpenDB(clickhouse.Connector(&clickhouse.Options{
    Addr:        []string{"localhost:9000"},
    Auth:        clickhouse.Auth{Database: "default", Username: "default"},
    DialTimeout: 10 * time.Second,
}))
// Set database/sql-only pool settings after creation
db.SetConnMaxIdleTime(5 * time.Minute)

Via context (per-query):

ctx := clickhouse.Context(context.Background(),
    clickhouse.WithQueryID("my-query-123"),
    clickhouse.WithSettings(clickhouse.Settings{"max_execution_time": 60}),
)
rows, err := conn.Query(ctx, "SELECT ...")

Connection options

Protocol and connection

Option	Type	Default	DSN param	Description	Best practice	When misconfigured
`Protocol`	`Protocol` (int)	`Native`	Scheme: `clickhouse://`=Native, `http://`=HTTP	Communication protocol: `Native` (0) for TCP, `HTTP` (1) for HTTP	Use Native for ~30% better performance. Use HTTP for proxy support, firewall traversal (port 80/443), or HTTP-only compression (`gzip`/`br`). See TCP vs HTTP.	HTTP scheme with Native port (9000): connection refused. Native blocked by firewall: timeouts.
`Addr`	`[]string`	`["localhost:9000"]` (Native) `["localhost:8123"]` (HTTP)	Comma-separated hosts in URL	List of `"host:port"` addresses for connection and failover	Specify multiple addresses in production for HA. Correct ports: 9000 (Native), 8123 (HTTP), 9440 (Native+TLS), 8443 (HTTP+TLS).	Single address: no failover. Wrong port: `"connection refused"`. Empty/nil: defaults to localhost, fails in distributed deployments.
`ConnOpenStrategy`	`ConnOpenStrategy` (uint8)	`ConnOpenInOrder` (0)	`connection_open_strategy` (`in_order`, `round_robin`, `random`)	Strategy for selecting a server from `Addr`. `InOrder` (0)=failover, `RoundRobin` (1)=load balance, `Random` (2)=random.	`InOrder` for active-standby. `RoundRobin` for active-active/K8s. `Random` to avoid thundering herd.	`InOrder` with active-active: first server gets all load, others idle. All strategies try all servers on failure — only affects which is tried first.

Authentication

Option	Type	Default	DSN param	Description	Best practice	When misconfigured
`Auth.Username`	`string`	`"default"`	`username` or URL user portion	Username for ClickHouse authentication	Never use `default` in production. Create dedicated users with minimal permissions.	Wrong username: `"Code: 516. DB::Exception: Authentication failed"`. Empty string: silently uses `"default"`.
`Auth.Password`	`string`	`""`	`password` or URL password portion	Password for ClickHouse authentication	Use env vars or secret managers in production. URL-encode special characters in DSN.	Wrong password: `"Code: 516. DB::Exception: Authentication failed"`. Special chars not URL-encoded: parsing errors.
`Auth.Database`	`string`	`""` (server default)	`database` or URL path (`/mydb`)	Default database for the connection	Always specify explicitly. Use dedicated databases per application in production.	Non-existent: `"Code: 81. DB::Exception: Database xyz doesn't exist"`. Empty in multi-tenant setup: queries hit wrong database.
`GetJWT`	`func(ctx) (string, error)`	`nil`	(programmatic only)	Callback returning JWT for ClickHouse Cloud auth. Overridable per query with `WithJWT(token)`. (Since v2.35.0)	Implement token caching/refresh - called per connection/request.	Expired token: auth errors. Blocking callback: timeouts. JWT takes precedence over user/pass. Requires TLS - without it, falls back to user/pass silently.

GetJWT: func(ctx context.Context) (string, error) {
    return getTokenFromVault(ctx)
}

Timeouts

Option	Type	Default	DSN param	Description	Best practice	When misconfigured
`DialTimeout`	`time.Duration`	`30s`	`dial_timeout`	Max time to establish a new connection. Also controls pool acquisition wait when `MaxOpenConns` is reached.	5-10s on LAN, 15-30s on WAN/cloud. Never below 1s.	Too short: `"clickhouse: acquire conn timeout"` during congestion. Too long (> 60s): app hangs during outages.
`ReadTimeout`	`time.Duration`	`5m` (300s)	`read_timeout`	Max time to wait for a server response per read call. Applied per block, not entire query. Context deadline takes precedence.	10-30s for short interactive queries; 5-30m for long analytical queries.	Too short: `"i/o timeout"` or `"read: connection reset by peer"` mid-query; server continues executing. Too long: dead connections not detected.

Connection pool

Option	Type	Default	DSN param	API	Description	Best practice	When misconfigured
`MaxIdleConns`	`int`	`5`	`max_idle_conns`	Both	Max idle (unused but alive) connections in pool	50-80% of expected concurrent queries. Low: 2-5, medium: 10-20, high: 20-50.	Too low: connection churn, higher latency. Too high: wasted memory. Capped at `MaxOpenConns` automatically.
`MaxOpenConns`	`int`	`MaxIdleConns + 5` (default: 10)	`max_open_conns`	Both	Max total connections (idle + active)	Low: 10-20, medium: 20-50, high: 50-100. Formula: concurrent queries + burst + buffer. Monitor: `SELECT * FROM system.metrics WHERE metric='TCPConnection'`.	Too low: `"clickhouse: acquire conn timeout"`. Too high: server `"Too many connections"`, FD limits exceeded. ClickHouse default `max_connections`: 1024 (shared).
`ConnMaxLifetime`	`time.Duration`	`1h`	`conn_max_lifetime`	Both	Max duration a connection can be reused. Checked on return to pool.	1-5h stable envs. 5-15m for K8s/rolling deploys. Never infinite.	Too short (< 1m): churn, higher latency. Too long/infinite: stale connections, DNS changes not picked up, traffic never rebalances.
`ConnMaxIdleTime`	`time.Duration`	`0` (none)	—	`database/sql` only	Max time a connection can sit idle before closing. Not in `Options` struct — set via `db.SetConnMaxIdleTime()`.	5-10m for K8s/bursty workloads to reclaim idle connections after traffic spikes.	Not set: idle connections persist until `ConnMaxLifetime`. Too short (< 30s): connections recreated during normal gaps.

database/sql onlyConnMaxIdleTime is a standard Go database/sql pool setting. It isn’t available in the clickhouse.Options struct or via clickhouse.Open(). Set it after OpenDB():

db := clickhouse.OpenDB(&clickhouse.Options{...})
db.SetConnMaxIdleTime(5 * time.Minute)

See Connection Pooling for usage details.

Standard database/sql pool settings

When using clickhouse.OpenDB() or sql.Open("clickhouse", dsn), the returned *sql.DB supports Go’s standard pool methods. OpenDB() auto-applies the first three from Options:

Method	Options equivalent	Notes
`db.SetMaxIdleConns(n)`	`MaxIdleConns`	Auto-applied by `OpenDB()`
`db.SetMaxOpenConns(n)`	`MaxOpenConns`	Auto-applied by `OpenDB()`
`db.SetConnMaxLifetime(d)`	`ConnMaxLifetime`	Auto-applied by `OpenDB()`
`db.SetConnMaxIdleTime(d)`	None	Must be set manually post-creation

ClickHouse API (clickhouse.Open)These methods are not available on the connection returned by clickhouse.Open(). The ClickHouse API manages its own pool internally using the Options struct fields directly.

Compression

Option	Type	Default	DSN param	Description	Best practice	When misconfigured
`Compression.Method`	`CompressionMethod` (byte)	None	`compress` (`lz4`, `zstd`, `lz4hc`, `gzip`, `deflate`, `br`, or `true` for LZ4)	Compression algorithm for data transfer. See protocol support matrix below.	LAN: None or LZ4. WAN: ZSTD or LZ4. CPU constrained: LZ4. Max compression: ZSTD (Native) or Brotli (HTTP). Skip for inserts < 1 MB.	GZIP/Brotli on Native: handshake failure. LZ4HC on HTTP: error or silent fallback. No compression on slow networks: 10-100x slower inserts.
`Compression.Level`	`int`	`3`	`compress_level`	Algorithm-specific intensity. GZIP/Deflate: -2 to 9. Brotli: 0 to 11. LZ4/ZSTD: ignored.	GZIP balanced: 3-6. Brotli balanced: 4-6.	Very high levels: extreme CPU, minimal benefit. Non-zero for LZ4/ZSTD: silently ignored. Level without compression enabled: no effect.
`MaxCompressionBuffer`	`int` (bytes)	`10485760` (10 MiB)	`max_compression_buffer`	Max compression buffer size before flushing. Each connection has its own buffer.	Default 10 MiB is good. 20-50 MiB for wide rows. Total memory = buffer x `MaxOpenConns`.	Too small (< 1 MiB): frequent flushes, poor efficiency. Too large (> 100 MiB): OOM with many connections.

Compression method support by protocol:

Method	Native	HTTP
`CompressionLZ4`	Yes	Yes
`CompressionLZ4HC`	Yes	No
`CompressionZSTD`	Yes	Yes
`CompressionGZIP`	No	Yes
`CompressionDeflate`	No	Yes
`CompressionBrotli`	No	Yes

TLS

Option	Type	Default	DSN param	Description	Best practice	When misconfigured
`TLS`	`*tls.Config`	`nil` (plain text)	`secure=true`, `skip_verify=true`	TLS/SSL config. Non-nil enables TLS. Ports: Native 9000/9440, HTTP 8123/8443.	Always enable in production and ClickHouse Cloud (required). `InsecureSkipVerify: false` in production. Add custom CAs via `RootCAs`.	Wrong port: `"connection reset by peer"`. `skip_verify=true` in prod: MITM vulnerable. Expired cert: `"x509: certificate has expired"`. Wrong host: `"x509: certificate is valid for X, not Y"`. Untrusted CA: `"x509: certificate signed by unknown authority"`. HTTP DSN with `secure=true`: use `https://` scheme instead.

See TLS for code examples.

Logging

Option	Type	Default	DSN param	Description	Best practice	When misconfigured
`Logger`	`*slog.Logger`	`nil` (no logging)	—	Structured logger via Go’s `log/slog`. Priority: `Debug`+`Debugf` > `Logger` > no-op. (Since v2.43.0)	Use `slog` with JSON handler in production. Add app context with `logger.With(...)`.	—
`Debug` (deprecated)	`bool`	`false`	`debug`	Legacy debug toggle. Use `Logger` instead. Logs to stdout unless `Debugf` is set.	—	Enabled in production: performance overhead, verbose logs, sensitive data in output.
`Debugf` (deprecated)	`func(string, ...any)`	`nil`	—	Custom debug log function. Use `Logger` instead. Requires `Debug: true`.	—	—

logger := slog.New(slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{Level: slog.LevelInfo}))
conn, err := clickhouse.Open(&clickhouse.Options{
    Logger: logger,
    // ...
})

See Logging for full examples.

Buffers and memory

Option	Type	Default	DSN param	Per-query	Description	Best practice	When misconfigured
`BlockBufferSize`	`uint8`	`2`	`block_buffer_size`	Yes (`WithBlockBufferSize`)	Decoded blocks to buffer when reading results. Enables concurrent read + decode.	Default 2 is fine. 5-10 for large streaming results. Memory = buffer x block size x concurrent queries.	Too small (1): blocks reader, higher latency. Too large (> 50): high memory, diminishing returns.
`FreeBufOnConnRelease`	`bool`	`false`	—	No	Release connection memory buffer after each query instead of reusing.	`false` for high query rates. `true` in memory-constrained containers or infrequent large batches.	`false` + limited memory: buffers accumulate (memory = buffer x idle conns). `true` + high rate: GC pressure, increased CPU.

HTTP-specific

Silently ignored on NativeThese options only affect Protocol: clickhouse.HTTP. They’re silently ignored when using the Native protocol, and no error or warning is emitted.

Option	Type	Default	DSN param	Description	Best practice	When misconfigured
`HttpHeaders`	`map[string]string`	`nil`	—	Additional HTTP headers on every request	Use for tracing (`X-Request-ID`), auth proxy headers. Keep minimal.	Overriding internal headers (`Content-Type`, `Authorization`): unpredictable behavior.
`HttpUrlPath`	`string`	`""`	`http_path`	URL path appended to requests. Leading `/` added automatically.	Use when behind reverse proxy with path routing.	Wrong path: HTTP 404 from proxy/LB.
`HttpMaxConnsPerHost`	`int`	`0` (unlimited)	—	TCP connections per host at transport layer (`http.Transport.MaxConnsPerHost`).	Leave at 0 for most apps. Only set when server has strict connection limits.	Too low (e.g., 10 with `MaxOpenConns`=50): transport bottleneck, slow queries despite low server load.
`HTTPProxyURL`	`*url.URL`	`nil` (uses env vars)	`http_proxy` (URL-encoded)	HTTP proxy for routing requests	Set explicitly if proxy required. Overrides `HTTP_PROXY`/`HTTPS_PROXY` env vars.	Wrong address: `"dial tcp: lookup proxy: no such host"`. Proxy needs auth: HTTP 407.
`TransportFunc`	`func(*http.Transport) (http.RoundTripper, error)`	`nil`	—	Custom HTTP transport factory. Receives default transport for wrapping. (Since v2.41.0)	Use for observability middleware. Don’t override `Proxy`, `DialContext`, `TLSClientConfig`.	Returning `nil`: panic. Overriding client fields: TLS/proxy silently ignored. Blocking RoundTripper: deadlocks.

Two-layer HTTP poolingWhen using HTTP, there are two connection pools:

Layer 1 (application): MaxIdleConns / MaxOpenConns — controls httpConnect objects
Layer 2 (transport): HttpMaxConnsPerHost — controls underlying TCP connections

The Native protocol has a simple 1:1 mapping and ignores HttpMaxConnsPerHost.

TransportFunc: func(t *http.Transport) (http.RoundTripper, error) {
    return &loggingRoundTripper{transport: t}, nil
}

Advanced connection

Option	Type	Default	DSN param	Description	Best practice	When misconfigured
`DialContext`	`func(ctx, addr) (net.Conn, error)`	`nil` (standard dialer)	—	Custom dial function for TCP connections. Works with both Native and HTTP.	Leave `nil` for 99% of cases. Use for Unix sockets, SOCKS proxy, custom DNS.	Not respecting context: hangs, resource leaks. With `TLS` set: custom dialer must handle TLS itself. Invalid `net.Conn`: crashes.
`DialStrategy`	`func(ctx, connID, options, dial) (DialResult, error)`	`DefaultDialStrategy`	—	Custom server selection and connection strategy. Overrides `ConnOpenStrategy`.	Use default for 99.9% of cases. Custom only for geo-aware routing, weighted selection, health checks.	Not trying all servers: fails with healthy servers available. Expensive ops inside: blocks pool acquisition on every connect.

Client information

Option	Type	Default	DSN param	Per-query	Description	Best practice	When misconfigured
`ClientInfo`	`ClientInfo` struct	Auto: `clickhouse-go` version + Go runtime	`client_info_product=myapp/1.0`	Yes (`WithClientInfo`, appends)	App identification sent to ClickHouse. Contains `Products` (`[]struct{Name,Version}`) and `Comment` (`[]string`). Visible in `system.query_log`.	Always set app name + version. Query attribution: `SELECT client_name FROM system.query_log WHERE client_name LIKE '%myapp%'`	Not setting: can’t identify which service issued queries in multi-service environments.

ClientInfo: clickhouse.ClientInfo{
    Products: []struct{ Name, Version string }{
        {Name: "my-service", Version: "1.0.0"},
    },
}
// Appears as: clickhouse-go/2.x my-service/1.0.0 (lv:go/1.23; os:linux)

ClickHouse server settings

Option	Type	Default	DSN param	Per-query	Description	Best practice	When misconfigured
`Settings`	`map[string]any`	`nil`	Any unrecognized param (e.g. `?max_execution_time=60`)	Yes (`WithSettings`, context wins on conflict)	ClickHouse server settings applied to every query. DSN conversion: `"true"`→`1`, `"false"`→`0`, numeric→`int`.	Set common limits at connection level, override per-query via context.	Typos: silently ignored or error by version. Wrong types: `"Cannot parse string 'abc' as Int64"`. `max_execution_time=0` + no deadline: queries run forever.
`CustomSetting`	`CustomSetting{Value string}`	—	—	Yes (via `WithSettings`)	Marks a setting as “custom” (non-important) for Native protocol. Won’t error if server doesn’t recognize it. HTTP treats all settings as custom by default.	Use for experimental or version-specific settings.	Marking important settings as custom: silently ignored if unsupported.

Common settings:

Setting	Type	Description
`max_execution_time`	int	Query timeout in seconds
`max_memory_usage`	int	Memory limit per query (bytes)
`max_block_size`	int	Block size for processing
`readonly`	int	1 = read-only, 2 = read-only + settings changes

Settings: clickhouse.Settings{
    "max_execution_time":  60,                                        // important -- errors if unknown
    "my_custom_setting":   clickhouse.CustomSetting{Value: "value"},  // custom -- ignored if unknown
}

Context-level query options

Set per-query using clickhouse.Context():

ctx := clickhouse.Context(context.Background(),
    clickhouse.WithQueryID("my-query"),
    clickhouse.WithSettings(clickhouse.Settings{"max_execution_time": 60}),
)

Context deadline behaviorIf the context has a deadline > 1s, max_execution_time is automatically set to seconds_remaining + 5. This overrides any manually set value.

Option	Type	Default	Protocol	Description	Best practice	When misconfigured
`WithQueryID`	`string`	Auto-generated	Both	Custom query identifier. Visible in `system.query_log` and `system.processes`.	Use UUIDs. Useful for `KILL QUERY WHERE query_id='...'`.	Duplicate IDs: confusion in `system.query_log`.
`WithQuotaKey`	`string`	`""`	Both	Quota key for multi-tenant resource limits. Requires server-side quota config.	Use for per-customer/per-user limits.	Quota not configured: silently ignored.
`WithJWT`	`string`	`""`	HTTPS only	Per-query JWT override for ClickHouse Cloud. (Since v2.35.0)	Use for per-request auth in multi-tenant proxies.	Without TLS: ignored, falls back to connection auth. Expired: `"Token has expired"`.
`WithSettings`	`Settings`	Inherits connection	Both	Per-query server settings. Merged with connection settings; context wins on conflict.	Override `max_execution_time` or `max_rows_to_read` per query type.	Same as connection-level `Settings`.
`WithParameters`	`Parameters` (`map[string]string`)	`nil`	Both	Server-side parameterized query values. Query syntax: `{param_name:Type}`.	Use instead of string concatenation for SQL injection safety.	Missing param: `"Substitution {param_name:Type} isn't set"`. Wrong type: `"Cannot parse string 'abc' as UInt64"`.
`WithAsync`	`bool` (wait)	Sync	Both	Async insert mode. Sets `async_insert=1`. `wait=true` adds `wait_for_async_insert=1`. Requires ClickHouse 21.11+. (Since v2.41.0; supersedes the older `WithStdAsync`.)	Use for high-throughput inserts.	`wait=false`: errors may be async — check `system.asynchronous_insert_log`. With SELECT: ignored. Old server: `"Unknown setting async_insert"`.
`WithLogs`	`func(*Log)`	`nil`	Native only	Server log entries callback during query execution.	Keep fast — blocks execution. Use goroutines for heavy processing.	On HTTP: silently never called.
`WithProgress`	`func(*Progress)`	`nil`	Native only	Query progress updates (rows/bytes processed).	Keep fast — blocks execution.	On HTTP: silently never called.
`WithProfileInfo`	`func(*ProfileInfo)`	`nil`	Native only	Query execution statistics callback.	Keep fast — blocks execution.	On HTTP: silently never called.
`WithProfileEvents`	`func([]ProfileEvent)`	`nil`	Native only	Performance counters callback.	Keep fast — blocks execution.	On HTTP: silently never called.
`WithoutProfileEvents`	—	Events sent	Native only	Suppress profile events. Performance optimization for servers ≥ 25.11. (Since v2.44.0)	Use when you don’t need profile events.	On older servers: error for unknown setting.
`WithExternalTable`	`...*ext.Table`	`nil`	Both	Attach temporary lookup tables to query. Data transferred per query.	Keep tables < 10 MB. Native more efficient than HTTP (multipart).	Large tables: network overhead per query.
`WithUserLocation`	`*time.Location`	Server timezone	Both	Override timezone for DateTime parsing.	Set explicitly when client/server timezones differ.	Wrong timezone: DateTime values silently off by hours, potential data corruption.
`WithColumnNamesAndTypes`	`[]ColumnNameAndType`	`nil` (runs DESCRIBE)	HTTP only	Skip `DESCRIBE TABLE` round trip on HTTP inserts by providing column info upfront. (Since v2.37.0)	Use when schema is known and stable.	Wrong types: `"Cannot convert String to UInt64"`. Schema drift after migration: stale info.
`WithBlockBufferSize`	`uint8`	Connection-level (2)	Both	Override connection-level `BlockBufferSize` for a single query.	Increase for large result sets on specific queries.	—
`WithClientInfo`	`ClientInfo`	Connection-level	Both	Append additional client info for a single query. Does not replace, appends. (Since v2.42.0)	Add per-request context (e.g., endpoint name).	—
`WithSpan`	`trace.SpanContext`	Empty	Native only	OpenTelemetry span context for distributed tracing.	See OpenTelemetry.	—

ctx := clickhouse.Context(ctx,
    clickhouse.WithQueryID("query-123"),
    clickhouse.WithParameters(clickhouse.Parameters{
        "user_id": "12345",
    }),
    clickhouse.WithProgress(func(p *clickhouse.Progress) {
        log.Printf("Progress: %d rows, %d bytes", p.Rows, p.Bytes)
    }),
)
rows, err := conn.Query(ctx, "SELECT * FROM users WHERE id = {user_id:String}")

Batch options

Passed to PrepareBatch(). Import: github.com/ClickHouse/clickhouse-go/v2/lib/driver.

Option	Default	Description	Best practice	When misconfigured
`WithReleaseConnection`	Connection held until `Send()`	Release connection to pool immediately after `PrepareBatch()`. Re-acquires on `Send()`/`Flush()`.	Use for long-lived batches (minutes/hours) to prevent pool exhaustion.	Not using for long batches: `"acquire conn timeout"` if many active.
`WithCloseOnFlush`	Batch stays open	Auto-close batch when `Flush()` is called.	Use for one-shot batches. Saves explicit `Close()`.	Using with multiple `Flush()` calls: first flush closes batch, subsequent ops fail.

batch, err := conn.PrepareBatch(ctx, "INSERT INTO table",
    driver.WithReleaseConnection(),
    driver.WithCloseOnFlush(),
)

Quick reference tables

Connection pool sizing recommendations

Application type	MaxIdleConns	MaxOpenConns	ConnMaxLifetime
Low-traffic web app	5	10	1h
Medium-traffic API	20	50	30m
High-traffic service	50	100	15m
Background batch jobs	10	20	2h
Kubernetes deployment	10	20	10m
Serverless (Lambda)	1	5	5m

Timeout recommendations

Environment	DialTimeout	ReadTimeout
Local / LAN	5s	30s
Cloud, same region	10s	2m
Cloud, cross region	30s	5m
OLAP workload	10s	30m
Realtime / OLTP	5s	10s

DSN parameter quick reference

DSN parameter	Options field	Example
`username`	`Auth.Username`	`?username=admin`
`password`	`Auth.Password`	`?password=secret`
`database`	`Auth.Database`	`?database=mydb` or `/mydb` in path
`dial_timeout`	`DialTimeout`	`?dial_timeout=10s`
`read_timeout`	`ReadTimeout`	`?read_timeout=5m`
`max_open_conns`	`MaxOpenConns`	`?max_open_conns=50`
`max_idle_conns`	`MaxIdleConns`	`?max_idle_conns=20`
`conn_max_lifetime`	`ConnMaxLifetime`	`?conn_max_lifetime=30m`
`connection_open_strategy`	`ConnOpenStrategy`	`?connection_open_strategy=round_robin`
`block_buffer_size`	`BlockBufferSize`	`?block_buffer_size=10`
`compress`	`Compression.Method`	`?compress=lz4`
`compress_level`	`Compression.Level`	`?compress_level=6`
`max_compression_buffer`	`MaxCompressionBuffer`	`?max_compression_buffer=20971520`
`secure`	`TLS`	`?secure=true`
`skip_verify`	`TLS.InsecureSkipVerify`	`?skip_verify=true`
`debug`	`Debug`	`?debug=true`
`client_info_product`	`ClientInfo.Products`	`?client_info_product=myapp/1.0`
`http_proxy`	`HTTPProxyURL`	`?http_proxy=http%3A%2F%2Fproxy%3A8080`
`http_path`	`HttpUrlPath`	`?http_path=/clickhouse`
(any other)	`Settings[key]`	`?max_execution_time=60`

Troubleshooting

Connection pool exhausted: “acquire conn timeout”

Cause: Connection pool exhausted - all MaxOpenConns connections are in use and none became available within DialTimeout. Fix Try the following steps in order, and diagnose the root cause before tuning knobs:

Check for long-running queries holding connections: SELECT query_id, elapsed FROM system.processes ORDER BY elapsed DESC. If found, address the slow queries first.
If you run long-lived batches (minutes/hours between PrepareBatch() and Send()), use WithReleaseConnection() to return the connection to the pool while the batch is open.
Increase MaxOpenConns to match observed concurrency.
Increase DialTimeout only if bursts are expected and acquisition wait is the actual bottleneck.

Read timeout and connection reset errors

Cause: ReadTimeout exceeded while waiting for a server response, or the connection was closed by the server/network. Fix:

Increase ReadTimeout for long-running queries
Use context deadlines for per-query timeout control
Check ClickHouse server-side max_execution_time limits

“Code: 516. Authentication failed”

Cause: Wrong username, password, or the user doesn’t exist. Fix:

Verify credentials against system.users table
Check for URL-encoding issues with special characters in DSN passwords
Confirm the user has access to the specified database

TLS certificate errors

Error	Cause	Fix
`x509: certificate has expired`	Server cert expired	Renew server certificate
`x509: certificate is valid for X, not Y`	Hostname mismatch	Use correct hostname or add to SANs
`x509: certificate signed by unknown authority`	Untrusted CA	Add CA to `tls.Config.RootCAs`
`connection reset by peer`	TLS/port mismatch	Use port 9440 (Native) or 8443 (HTTP) for TLS

Gradual memory growth

Cause: Large idle connection buffers accumulating. Fix:

Set FreeBufOnConnRelease: true in memory-constrained environments
Reduce MaxIdleConns to limit idle connections
Reduce MaxCompressionBuffer if using compression
Lower ConnMaxLifetime to cycle connections more frequently

Third-party clients

Documentation Index

​How options are set

​Connection options

​Protocol and connection

​Authentication

​Timeouts

​Connection pool

​Standard database/sql pool settings

​Compression

​TLS

​Logging

​Buffers and memory

​HTTP-specific

​Advanced connection

​Client information

​ClickHouse server settings

​Context-level query options

​Batch options

​Quick reference tables

​Connection pool sizing recommendations

​Timeout recommendations

​DSN parameter quick reference

​Troubleshooting

​Connection pool exhausted: “acquire conn timeout”

​Read timeout and connection reset errors

​“Code: 516. Authentication failed”

​TLS certificate errors

​Gradual memory growth

How options are set

Connection options

Protocol and connection

Authentication

Timeouts

Connection pool

Standard database/sql pool settings

Compression

TLS

Logging

Buffers and memory

HTTP-specific

Advanced connection

Client information

ClickHouse server settings

Context-level query options

Batch options

Quick reference tables

Connection pool sizing recommendations

Timeout recommendations

DSN parameter quick reference

Troubleshooting

Connection pool exhausted: “acquire conn timeout”

Read timeout and connection reset errors

“Code: 516. Authentication failed”

TLS certificate errors

Gradual memory growth