How to Adjust ClickHouse Connection Timeout in PostgreSQL

Galaxy Glossary

How do I change the ClickHouse connection timeout when querying from PostgreSQL?

The ClickHouse connection timeout sets how long a client (e.g., clickhouse_fdw in PostgreSQL) waits to establish a connection before aborting.

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Description

Why change the ClickHouse connection timeout?

Slow networks, VPN hops, or overloaded ClickHouse nodes can delay hand-shakes. Raising the timeout prevents false failures, while lowering it surfaces real issues quickly.

What is the default timeout?

The native clickhouse-client defaults to 10 000 ms (10 s). Most drivers, including clickhouse_fdw and the HTTP interface, inherit the same default unless overridden.

How do I set timeout when creating a foreign server?

PostgreSQL CREATE SERVER

Pass connect_timeout in the OPTIONS clause. The value is in seconds.

How do I modify the timeout later?

Use ALTER SERVER to replace the option without dropping objects. Afterwards, reconnect sessions or run DISCARD ALL.

How can I verify the current timeout?

Query pg_foreign_server or run \des+ in psql. Look for the connect_timeout key.

What about other drivers?

• Native client: --connect_timeout
• https: add timeout=5 URL param
• JDBC: socket_timeout=5000 (ms)

Best practices for production

• Keep timeouts symmetrical across read & write paths.
• Log failed attempts with duration to tune later.
• Combine with statement_timeout in PostgreSQL to guard long-running queries.

Why How to Adjust ClickHouse Connection Timeout in PostgreSQL is important

How to Adjust ClickHouse Connection Timeout in PostgreSQL Example Usage


-- Quick smoke test that respects the 5-second timeout
SELECT id, name, email
FROM customers
WHERE created_at > now() - INTERVAL '30 days'
LIMIT 50;

How to Adjust ClickHouse Connection Timeout in PostgreSQL Syntax


-- Create a ClickHouse foreign server with explicit 5-second timeout
CREATE SERVER ch_srv
  FOREIGN DATA WRAPPER clickhouse_fdw
  OPTIONS (
    host 'clickhouse.local',
    port '8123',
    dbname 'ecom',
    connect_timeout '5'  -- seconds
  );

-- Map a foreign table in the familiar ecommerce schema
CREATE FOREIGN TABLE customers (
  id         UInt64,
  name       String,
  email      String,
  created_at DateTime
) SERVER ch_srv OPTIONS (table_name 'customers');

Common Mistakes

Using milliseconds in clickhouse_fdw. The option expects seconds. Supplying 5000 produces a 5 000-second wait, not 5 s. Always use integer seconds.
Altering the server but keeping old sessions alive. PostgreSQL caches FDW connections per session. Disconnect or run DISCARD ALL to pick up new settings.

Frequently Asked Questions (FAQs)

Does changing connect_timeout affect running queries?

No. It only impacts future connection attempts. Cancel and rerun active queries if you need the new value.

Can I set different timeouts per user?

Yes. Create multiple foreign servers (e.g., ch_fast, ch_slow) with different connect_timeout values and grant usage selectively.

Is there a maximum safe value?

ClickHouse itself imposes no hard cap, but values above typical TCP keep-alive (2 min) rarely help. Start at 10-30 s and adjust based on logs.

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