{"Entry":{"collection":"plan","key":"gather","name":"Gather","aliases":["Gather","T_Gather"],"metadata":{"aliases":["Gather","T_Gather"],"category":"Parallelism","content_hash":"bc5c10ef300616be8410f15b621a14789ef9325912f54a4d163750740b3678c5","imported_at":"2026-09-30T00:40:44.191527+08:00","name":"Gather","name_zh":"Gather","slug":"gather","summary":"Combines tuples from parallel workers and any participating leader execution."}},"Definition":{"Collection":"plan","Key":"gather","SourceDatabase":"center","Version":"18","SourceTable":"plan_node","SourceKey":"gather","SourceRevision":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","Facts":{"comparison_data":{"initializer":"ExecInitGather","memory_mechanism":"unclassified","node_tag":"T_Gather","parallel_callbacks":[],"partial_modes":[],"strategies":[],"text_names":["Gather"]},"comparison_hash":"6fbd3ac9f7150ee72d4ae315dded3c145ee461d926c8ce6a04a1b8c6c31deee4","description":["Combines tuples from parallel workers and any participating leader execution."],"evidence_kind":"source and documentation","explain_names":["Gather"],"explain_prefixes":["Parallel","Async"],"facts":[{"label":"Core node tag","value":"T_Gather"},{"label":"Structured EXPLAIN Node Type","value":"Gather"},{"label":"Inputs","value":"One parallel child plan"},{"label":"Output","value":"Combined tuples without a merge-order guarantee"},{"label":"Executor initializer","value":"ExecInitGather"},{"label":"Memory mechanism","value":"unclassified"}],"memory":{"description":"This extraction does not assign a universal memory limit or spill policy to this node. Inspect the same-build implementation and its expressions or provider.","evidence":[{"archive_sha256":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","label":"src/backend/executor/nodeGather.c","path":"src/backend/executor/nodeGather.c","sha256":"5dd854fbee33c36b3141f3297971f07aeea83a2a83c24cd99a03726fdf68bbb7","url":"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2"}],"mechanism":"unclassified","source_notes":[]},"node_tag":"T_Gather","parallel_callbacks":[],"partial_modes":[],"related":[{"label":"EXPLAIN","url":"/wiki/sql/explain/?v=18"},{"label":"Using EXPLAIN","url":"/docs/18/using-explain.html"},{"label":"Parallel plans","url":"/docs/18/parallel-plans.html"},{"label":"max_parallel_workers_per_gather","url":"/wiki/guc/max_parallel_workers_per_gather/?v=18"},{"label":"parallel_leader_participation","url":"/wiki/guc/parallel_leader_participation/?v=18"}],"release":{"channel":"stable","label":"18.6","major":"18","ref":"PostgreSQL 18.6 source archive","revision":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","source_snapshot_utc":"","source_url":"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2"},"runtime_verified":false,"sections":[{"paragraphs":["Structured formats use the Node Type above. Text-format spellings can also include operation, strategy, join type, scan direction or aggregation-stage attributes.","Text names recorded by this source: Gather.","Parallel-aware and parallel-safe are different plan properties. A node running inside a parallel worker is not necessarily a parallel-aware node."],"title":"EXPLAIN names and attributes"},{"paragraphs":["This extraction does not assign a universal memory limit or spill policy to this node. Inspect the same-build implementation and its expressions or provider."],"title":"Memory and temporary storage"},{"paragraphs":["The source callbacks below can coordinate execution or collect worker instrumentation. Their presence is not a blanket claim that this node supports a shared parallel scan or shared state.","Callbacks in this build: none extracted from this node implementation."],"title":"Parallel execution and instrumentation"},{"paragraphs":["PostgreSQL supports parallel aggregation by aggregating in two stages. First, each process participating in the parallel portion of the query performs an aggregation step, producing a partial result for each group of which that process is aware. This is reflected in the plan as a Partial Aggregate node. Second, the partial results are transferred to the leader via Gather or Gather Merge . Finally, the leader re-aggregates the results across all workers in order to produce the final result. This is reflected in the plan as a Finalize Aggregate node."],"title":"Same-version manual discussion"},{"paragraphs":["nodeGather.c Support routines for scanning a plan via multiple workers.","A Gather executor launches parallel workers to run multiple copies of a plan. It can also run the plan itself, if the workers are not available or have not started up yet. It then merges all of the results it produces and the results from the workers into a single output stream. Therefore, it will normally be used with a plan where running multiple copies of the same plan does not produce duplicate output, such as parallel-aware SeqScan.","Alternatively, a Gather node can be configured to use just one worker and the single-copy flag can be set. In this case, the Gather node will run the plan in one worker and will not execute the plan itself. In this case, it simply returns whatever tuples were returned by the worker. If a worker cannot be obtained, then it will run the plan itself and return the results. Therefore, a plan used with a single-copy Gather node need not be parallel-aware.","Leader may access ExecProcNode result directly (if need_to_scan_locally), or from workers via tuple queue. So we can't trivially rely on the slot type being fixed for expressions evaluated within this node.","Without projections result slot type is not trivially known, see comment above."],"title":"Executor implementation notes"},{"code":"case T_Gather:\n\t\t\tpname = sname = \"Gather\";\n\t\t\tbreak;","title":"EXPLAIN identity in core source"}],"source_inventory":{"executor":"src/backend/executor/execProcnode.c","explain":"src/backend/commands/explain.c","implementation":"src/backend/executor/nodeGather.c"},"sources":[{"archive_sha256":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","label":"src/backend/commands/explain.c:1438","line":1438,"path":"src/backend/commands/explain.c","sha256":"34c86d6070224a0e981efef51f79101d6d505e5874f1684ace183034bab14bb4","url":"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2"},{"archive_sha256":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","label":"src/backend/executor/execProcnode.c:355","line":355,"path":"src/backend/executor/execProcnode.c","sha256":"f8a06a3f539077249b20664b2812433db6d7bd12b2c0ca633525db43d06f112a","url":"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2"},{"archive_sha256":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","label":"src/backend/executor/nodeGather.c","path":"src/backend/executor/nodeGather.c","sha256":"5dd854fbee33c36b3141f3297971f07aeea83a2a83c24cd99a03726fdf68bbb7","url":"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2"},{"archive_sha256":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","label":"src/include/nodes/plannodes.h","path":"src/include/nodes/plannodes.h","sha256":"52422b327a8049fbbb20d8b96008a0fc0a6fafa60f7eff3c695d5b2e83830120","url":"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2"},{"label":"PostgreSQL 18.6 · parallel-plans","path":"parallel-plans.html","sha256":"62207d207bead82b01b59dc119c4f95856f08655cc11d4699a05a40867ed2070","url":"/docs/18/parallel-plans.html#PARALLEL-AGGREGATION"}],"strategies":[],"tables":[{"columns":[{"key":"label","label":"Text-format label"},{"key":"identity","label":"Structured node identity"}],"key":"explain-labels","rows":[{"identity":"Gather","label":"Gather"}],"title":"EXPLAIN labels in this source build"}]},"ManualEvidence":{"release":{"channel":"stable","label":"18.6","major":"18","ref":"PostgreSQL 18.6 source archive","revision":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","source_snapshot_utc":"","source_url":"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2"},"sources":[{"archive_sha256":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","label":"src/backend/commands/explain.c:1438","line":1438,"path":"src/backend/commands/explain.c","sha256":"34c86d6070224a0e981efef51f79101d6d505e5874f1684ace183034bab14bb4","url":"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2"},{"archive_sha256":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","label":"src/backend/executor/execProcnode.c:355","line":355,"path":"src/backend/executor/execProcnode.c","sha256":"f8a06a3f539077249b20664b2812433db6d7bd12b2c0ca633525db43d06f112a","url":"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2"},{"archive_sha256":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","label":"src/backend/executor/nodeGather.c","path":"src/backend/executor/nodeGather.c","sha256":"5dd854fbee33c36b3141f3297971f07aeea83a2a83c24cd99a03726fdf68bbb7","url":"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2"},{"archive_sha256":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","label":"src/include/nodes/plannodes.h","path":"src/include/nodes/plannodes.h","sha256":"52422b327a8049fbbb20d8b96008a0fc0a6fafa60f7eff3c695d5b2e83830120","url":"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2"},{"label":"PostgreSQL 18.6 · parallel-plans","path":"parallel-plans.html","sha256":"62207d207bead82b01b59dc119c4f95856f08655cc11d4699a05a40867ed2070","url":"/docs/18/parallel-plans.html#PARALLEL-AGGREGATION"}]},"MeasuredEvidence":{"runtime_verified":false}},"Text":{"Collection":"plan","Key":"gather","SourceDatabase":"center","Version":"18","Locale":"en","Title":"Gather","Summary":"Combines tuples from parallel workers and any participating leader execution.","BodyHTML":"\u003cp\u003eCombines tuples from parallel workers and any participating leader execution.\u003c/p\u003e","SourceRevision":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","ContentHash":"87a5e9f2b860927ca3485fd8ccf5814d329e15c04f8ac7494cda26becca46fcc","Payload":{"description":["Combines tuples from parallel workers and any participating leader execution."],"related":[{"label":"EXPLAIN","url":"/wiki/sql/explain/?v=18"},{"label":"Using EXPLAIN","url":"/docs/18/using-explain.html"},{"label":"Parallel plans","url":"/docs/18/parallel-plans.html"},{"label":"max_parallel_workers_per_gather","url":"/wiki/guc/max_parallel_workers_per_gather/?v=18"},{"label":"parallel_leader_participation","url":"/wiki/guc/parallel_leader_participation/?v=18"}],"sections":[{"paragraphs":["Structured formats use the Node Type above. Text-format spellings can also include operation, strategy, join type, scan direction or aggregation-stage attributes.","Text names recorded by this source: Gather.","Parallel-aware and parallel-safe are different plan properties. A node running inside a parallel worker is not necessarily a parallel-aware node."],"title":"EXPLAIN names and attributes"},{"paragraphs":["This extraction does not assign a universal memory limit or spill policy to this node. Inspect the same-build implementation and its expressions or provider."],"title":"Memory and temporary storage"},{"paragraphs":["The source callbacks below can coordinate execution or collect worker instrumentation. Their presence is not a blanket claim that this node supports a shared parallel scan or shared state.","Callbacks in this build: none extracted from this node implementation."],"title":"Parallel execution and instrumentation"},{"paragraphs":["PostgreSQL supports parallel aggregation by aggregating in two stages. First, each process participating in the parallel portion of the query performs an aggregation step, producing a partial result for each group of which that process is aware. This is reflected in the plan as a Partial Aggregate node. Second, the partial results are transferred to the leader via Gather or Gather Merge . Finally, the leader re-aggregates the results across all workers in order to produce the final result. This is reflected in the plan as a Finalize Aggregate node."],"title":"Same-version manual discussion"},{"paragraphs":["nodeGather.c Support routines for scanning a plan via multiple workers.","A Gather executor launches parallel workers to run multiple copies of a plan. It can also run the plan itself, if the workers are not available or have not started up yet. It then merges all of the results it produces and the results from the workers into a single output stream. Therefore, it will normally be used with a plan where running multiple copies of the same plan does not produce duplicate output, such as parallel-aware SeqScan.","Alternatively, a Gather node can be configured to use just one worker and the single-copy flag can be set. In this case, the Gather node will run the plan in one worker and will not execute the plan itself. In this case, it simply returns whatever tuples were returned by the worker. If a worker cannot be obtained, then it will run the plan itself and return the results. Therefore, a plan used with a single-copy Gather node need not be parallel-aware.","Leader may access ExecProcNode result directly (if need_to_scan_locally), or from workers via tuple queue. So we can't trivially rely on the slot type being fixed for expressions evaluated within this node.","Without projections result slot type is not trivially known, see comment above."],"title":"Executor implementation notes"},{"code":"case T_Gather:\n\t\t\tpname = sname = \"Gather\";\n\t\t\tbreak;","title":"EXPLAIN identity in core source"}],"tables":[{"columns":[{"key":"label","label":"Text-format label"},{"key":"identity","label":"Structured node identity"}],"key":"explain-labels","rows":[{"identity":"Gather","label":"Gather"}],"title":"EXPLAIN labels in this source build"}]}},"RequestedLocale":"zh-Hans","Fallback":true,"Versions":["10","11","12","13","14","15","16","17","18","19","20"],"Locales":["en"],"Signatures":null,"Spellings":null,"SQLState":null,"Evidence":null}
