Cookbook recipe

Subtransactions and the multixact cost

Applies to PostgreSQL 13–17 Last reviewed May 2026 Grounded in source

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A production experience capsule based on real incident patterns.

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Estimated investigation4 min

Scenario

Heavy use of savepoints/PL functions with exception blocks can cause mysterious slowdowns from SLRU/multixact contention. Diagnose it Watch for SLRU-related waits: SELECT wait_event, count(*) FROM pg_stat_activity WHERE wait_event LIKE '%SLRU%' OR wait_event LIKE '%MultiXact%' GROUP BY…

Investigation Path

Heavy use of savepoints/PL functions with exception blocks can cause mysterious slowdowns from SLRU/multixact contention.

Diagnose it

Watch for SLRU-related waits:

SELECT wait_event, count(*) FROM pg_stat_activity
WHERE wait_event LIKE '%SLRU%' OR wait_event LIKE '%MultiXact%'
GROUP BY 1;

Why it happens

Each savepoint or PL/pgSQL EXCEPTION block opens a subtransaction with its own XID. Many concurrent row lockers create multixacts. Both rely on SLRU caches that can become contended at scale.

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Concepts on this page

Transaction Savepoint
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