Connection pooling

Last reviewed Jun 1, 2026 Content v20260601

Track mode

sql_sandbox

Means

SQL sandbox

Reading

~1 min

Level

intermediate

This lesson

This lesson teaches Connection pooling: the SQL patterns, schema habits, and query reasoning you need before advancing in MySQL.

Teams query Connection pooling on every MySQL codebase—skipping it leaves gaps in debugging and data reviews.

You will apply Connection pooling in contexts like: Web apps on shared hosting, ecommerce, and many startups’ first production DB.

Copy MySQL SQL into the mysql client, local MySQL/MariaDB, or DB Fiddle (MySQL dialect)—use DESCRIBE and EXPLAIN where lessons show them. The in-browser lab ships later; mysql client is the practice path now.

Toward the end—consolidate before MariaDB notes, interview prep, and production checklist.

Opening a connection per request is expensive—PHP-FPM + persistent PDO, ProxySQL, or RDS Proxy pool connections at scale.

PHP perspective

Laravel reuses connections per request lifecycle; avoid creating new PDO in tight loops. Tune max_connections on server vs pool size × app instances.

Too many connections

SHOW STATUS LIKE 'Threads_connected';
SHOW VARIABLES LIKE 'max_connections';

Timeouts

Set wait_timeout and app idle timeouts so leaked connections do not exhaust the server.

Important interview questions and answers

Q: max_connections exceeded?
A: Too many app servers or leaks—add pooler or fix connection closing.
Q: Persistent PDO?
A: Can reduce handshake cost—watch stale connection handling.

Self-check

What status shows active connections?
Why pool at proxy layer?

Tip: Watch Threads_connected vs max_connections during load tests.

Interview prep

Too many connections?: Scale pooler or reduce per-server connections.
Threads_connected?: Status variable for open client threads.

Discussion

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Starter discussion topics

max_connections?
PDO persistent?

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