Merge Append

Description

Merges sorted child streams while preserving the required ordering.

Core node tag

T_MergeAppend

Structured EXPLAIN Node Type

Merge Append

Inputs

Multiple sorted child plans

Output

Merged ordered tuples

Executor initializer

ExecInitMergeAppend

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: Merge Append.

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.

Executor implementation notes

NOTES A MergeAppend node contains a list of one or more subplans. These are each expected to deliver tuples that are sorted according to a common sort key. The MergeAppend node merges these streams to produce output sorted the same way.

MergeAppend nodes don't make use of their left and right subtrees, rather they maintain a list of subplans so a typical MergeAppend node looks like this in the plan tree:

... / MergeAppend---+------+------+--- nil / \ | | | nil nil ... ... ... subplans

We have one slot for each item in the heap array. We use SlotNumber to store slot indexes. This doesn't actually provide any formal type-safety, but it makes the code more self-documenting.

If run-time partition pruning is enabled, then set that up now

EXPLAIN identity in core source

case T_MergeAppend:
			pname = sname = "Merge Append";
			break;

Related entries

• EXPLAIN
• Using EXPLAIN
• Parallel plans

Documentation and source

• src/backend/commands/explain.c:1409
• src/backend/executor/execProcnode.c:186
• src/backend/executor/nodeMergeAppend.c
• src/include/nodes/plannodes.h

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

• AppendAppend
• Recursive UnionRecursiveUnion
• SetOpSetOp
• UniqueUnique