Group

Description

Groups an ordered input stream by its grouping columns.

Core node tag

T_Group

Structured EXPLAIN Node Type

Group

Inputs

One ordered child plan

Output

One tuple per group

Executor initializer

ExecInitGroup

Memory mechanism

unclassified

EXPLAIN names and attributes

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: Group.

Parallel-aware and parallel-safe are different plan properties. A node running inside a parallel worker is not necessarily a parallel-aware node.

Memory and temporary storage

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.

Parallel execution and instrumentation

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.

Same-version manual discussion

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.

Executor implementation notes

nodeGroup.c Routines to handle group nodes (used for queries with GROUP BY clause).

DESCRIPTION The Group node is designed for handling queries with a GROUP BY clause. Its outer plan must deliver tuples that are sorted in the order specified by the grouping columns (ie. tuples from the same group are consecutive). That way, we just have to compare adjacent tuples to locate group boundaries.

The ScanTupleSlot holds the (copied) first tuple of each group.

We need not call ResetExprContext here because ExecQualAndReset() will reset the per-tuple memory context once per input tuple.

If first time through, acquire first input tuple and determine whether to return it or not.

EXPLAIN identity in core source

case T_Group:
			pname = sname = "Group";
			break;

Related entries

• EXPLAIN
• Using EXPLAIN
• Parallel plans

Documentation and source

• src/backend/commands/explain.c:1528
• src/backend/executor/execProcnode.c:335
• src/backend/executor/nodeGroup.c
• src/include/nodes/plannodes.h
• PostgreSQL 18.6 · parallel-plans

Compare versions

PostgreSQL 17 → 18: unchanged.

Compares recorded interfaces and attributes. Source fingerprints and build metadata are excluded; an absent sample is not proof of the introduction or removal release.

Related entries

• AggregateAgg
• WindowAggWindowAgg