Multiplier applied to work_mem for hash-based operations.
At a glance
| Property | Value |
|---|---|
| Parameter | hash_mem_multiplier |
| Category | Memory |
| Default | 2.0 |
| Value type | boolean / enum / numeric |
| Change scope | Per-session (SET) |
What it does
Hash operations may use work_mem x hash_mem_multiplier before spilling. Default is 2.0 since PostgreSQL 15, reflecting that hashes benefit more from memory than sorts.
How to apply a change
Can be set per session with SET, per role/database with ALTER ROLE/DATABASE ... SET, or globally in postgresql.conf.
Inspect the current value and source with SHOW hash_mem_multiplier; or SELECT name, setting, unit, context, source FROM pg_settings WHERE name = 'hash_mem_multiplier';.
Tuning guidance
Raise (e.g. 3.0-4.0) when hash joins/aggregates spill to disk but you do not want to inflate sort memory. Pair with monitoring of temp-file usage.