{"Entry":{"collection":"guc","key":"enable_mergejoin","name":"enable_mergejoin","aliases":[],"metadata":{"baseline":true,"boot_human":"Not specified","boot_val":null,"category":"Query Tuning / Planner Method Configuration","category_zh":"","changed_in":[],"changes":[{"documentation_changed":true,"fields":{},"from":"7.4","status":"changed","to":"8.0"}],"content_hash":"d6042e111474cf934361373b5eb64d8b52f65219980292c0b19b46cace9e02bb","context":"","default_changed_in":[],"default_history":[{"from":"9.0","to":"19","value":"on"}],"editorial":{"advice":{"olap":"Benchmark representative analytical plans with and without enable_mergejoin. Judge planning time, memory, spill, and execution time together; a win for one report is not evidence for a global setting.","oltp":"Keep the upstream default for routine OLTP. Use a transaction- or session-local change to diagnose whether choosing merge joins, which consume inputs ordered on compatible join keys explains a regression, then repair statistics, indexes, estimates, or query shape instead of leaving a cluster-wide method ban.","small":"Do not use enable_mergejoin as a permanent hint on a small host. Resource pressure may change the best method, so verify with EXPLAIN (ANALYZE, BUFFERS) and keep the change scoped to the affected workload if it is still needed."},"mechanism":["enable_mergejoin controls whether the planner may choose merge joins, which consume inputs ordered on compatible join keys.","It is consulted while a plan is built. A session-level change is useful for comparing EXPLAIN alternatives, but an already cached plan is not retroactively rewritten; invalidation or replanning is required to observe a different choice.","The switch changes which candidate paths the planner may cost; it does not make the chosen method faster by itself. Its user context permits session- or transaction-local changes; newly performed or newly planned work sees the value."],"pitfalls":["Treating enable_mergejoin as a query hint even though it affects every plan built in its scope.","Testing an already cached prepared plan and concluding that the setting has no effect.","Masking stale statistics or cardinality errors by disabling a plan method globally.","The switch changes which candidate paths the planner may cost; it does not make the chosen method faster by itself."],"references":[{"title":"PostgreSQL 19 Beta 4: enable_mergejoin","url":"https://www.postgresql.org/docs/19/runtime-config-query.html#GUC-ENABLE-MERGEJOIN"},{"title":"PostgreSQL 19 release notes","url":"https://www.postgresql.org/docs/19/release-19.html"}],"related":["enable_hashjoin","enable_nestloop","enable_sort","enable_indexscan","work_mem"],"summary":"enable_mergejoin — Enables the planner's use of merge join plans. Observed in PG9.0–19 Beta 4; its last measured boot default is on in PG19 Beta 4, with user context. This is a beta-snapshot fact and can change before PostgreSQL 19 GA."},"enumvals":[],"first_version":"7.4","group":"Query Tuning","group_slug":"query","imported_at":"2026-09-27T17:57:31.210027+08:00","intro_commit":{},"key":"enable_mergejoin","last_version":"20","max_val":"","min_val":"","name":"enable_mergejoin","position":126,"present_in":["7.4","8.0","8.1","8.2","8.3","8.4","9.0","9.1","9.2","9.3","9.4","9.5","9.6","10","11","12","13","14","15","16","17","18","19","20"],"short_desc":"Enables or disables the query planner's use of merge-join plan types.","short_desc_zh":"","source_rev":"english-manuals:5062f8cf89c2d086ff0dc74e235fb8be2f5ae43fb6d95281a086f311dff32033","unit":"","vartype":"bool"}},"Definition":{"Collection":"guc","Key":"enable_mergejoin","SourceDatabase":"center","Version":"18","SourceTable":"guc","SourceKey":"enable_mergejoin","SourceRevision":"english-manuals:5062f8cf89c2d086ff0dc74e235fb8be2f5ae43fb6d95281a086f311dff32033","Facts":{"boot_val":"on","category":"Query Tuning / Planner Method Configuration","context":"user","description":"Enables or disables the query planner's use of merge-join plan types. The default is on.","doc":{"anchor":"GUC-ENABLE-MERGEJOIN","file":"runtime-config-query.html","lang":"en","sha256":"33456ca7ef52bf28e93e34b932a411db7a1b95800179add2cf867dc6ec9144bc","slug":"18"},"documented":true,"enumvals":null,"extra_desc":null,"lang":"en","max_val":null,"metadata_version":"18","min_val":null,"name":"enable_mergejoin","short_desc":"Enables the planner's use of merge join plans.","source":"pg-settings-source-snapshot","unit":null,"vartype":"bool"},"ManualEvidence":{"doc":{"anchor":"GUC-ENABLE-MERGEJOIN","file":"runtime-config-query.html","lang":"en","sha256":"33456ca7ef52bf28e93e34b932a411db7a1b95800179add2cf867dc6ec9144bc","slug":"18"}},"MeasuredEvidence":{"metadata_version":"18"}},"Text":{"Collection":"guc","Key":"enable_mergejoin","SourceDatabase":"center","Version":"18","Locale":"en","Title":"enable_mergejoin","Summary":"Enables or disables the query planner's use of merge-join plan types. The default is on.","BodyHTML":"\u003cp\u003eEnables or disables the query planner\u0026#39;s use of merge-join plan types. The default is on.\u003c/p\u003e","SourceRevision":"english-manuals:5062f8cf89c2d086ff0dc74e235fb8be2f5ae43fb6d95281a086f311dff32033","ContentHash":"173a13c18866d814183f98b160a91415fe4b532b5079a49d90abd87aa05fd848","Payload":{"description":"Enables or disables the query planner's use of merge-join plan types. The default is on."}},"RequestedLocale":"zh-Hans","Fallback":true,"Versions":["10","11","12","13","14","15","16","17","18","19","20","7.4","8.0","8.1","8.2","8.3","8.4","9.0","9.1","9.2","9.3","9.4","9.5","9.6"],"Locales":["en"],"Signatures":null,"Spellings":null,"SQLState":null,"Evidence":null}