Custom Scan

Description

Executes a plan supplied by a custom-scan provider.

Core node tag

T_CustomScan

Structured EXPLAIN Node Type

Custom Scan

Inputs

Provider-defined plan, possibly with children

Output

Provider-defined tuples

Executor initializer

ExecInitCustomScan

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: Custom Scan (%s).

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: ExecCustomScanEstimate, ExecCustomScanInitializeDSM, ExecCustomScanInitializeWorker, ExecCustomScanReInitializeDSM.

Executor implementation notes

nodeCustom.c Routines to handle execution of custom scan node

Allocate the CustomScanState object. We let the custom scan provider do the palloc, in case it wants to make a larger object that embeds CustomScanState as the first field. It must set the node tag and the methods field correctly at this time. Other standard fields should be set to zero.

Use a custom slot if specified in CustomScanState or use virtual slot otherwise.

Determine the scan tuple type. If the custom scan provider provided a targetlist describing the scan tuples, use that; else use base relation's rowtype.

The callback of custom-scan provider applies the final initialization of the custom-scan-state node according to its logic.

EXPLAIN identity in core source

case T_CustomScan:
			sname = "Custom Scan";
			custom_name = ((CustomScan *) plan)->methods->CustomName;
			if (custom_name)
				pname = psprintf("Custom Scan (%s)", custom_name);
			else
				pname = sname;
			break;

Related entries

• EXPLAIN
• Using EXPLAIN
• Parallel plans

Documentation and source

• src/backend/commands/explain.c:1508
• src/backend/executor/execProcnode.c:289
• src/backend/executor/nodeCustom.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

• Foreign ScanForeignScan