postgres/src/backend/executor/execScan.c

/*-------------------------------------------------------------------------
 *
 * execScan.c
 *	  This code provides support for generalized relation scans. ExecScan
 *	  is passed a node and a pointer to a function to "do the right thing"
 *	  and return a tuple from the relation. ExecScan then does the tedious
 *	  stuff - checking the qualification and projecting the tuple
 *	  appropriately.
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *	  src/backend/executor/execScan.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "executor/executor.h"
#include "executor/execScan.h"
#include "miscadmin.h"

/* ----------------------------------------------------------------
 *		ExecScan
 *
 *		Scans the relation using the 'access method' indicated and
 *		returns the next qualifying tuple.
 *		The access method returns the next tuple and ExecScan() is
 *		responsible for checking the tuple returned against the qual-clause.
 *
 *		A 'recheck method' must also be provided that can check an
 *		arbitrary tuple of the relation against any qual conditions
 *		that are implemented internal to the access method.
 *
 *		Conditions:
 *		  -- the "cursor" maintained by the AMI is positioned at the tuple
 *			 returned previously.
 *
 *		Initial States:
 *		  -- the relation indicated is opened for scanning so that the
 *			 "cursor" is positioned before the first qualifying tuple.
 * ----------------------------------------------------------------
 */
TupleTableSlot *
ExecScan(ScanState *node,
		 ExecScanAccessMtd accessMtd,	/* function returning a tuple */
		 ExecScanRecheckMtd recheckMtd)
{
	EPQState   *epqstate;
	ExprState  *qual;
	ProjectionInfo *projInfo;

	epqstate = node->ps.state->es_epq_active;
	qual = node->ps.qual;
	projInfo = node->ps.ps_ProjInfo;

	return ExecScanExtended(node,
							accessMtd,
							recheckMtd,
							epqstate,
							qual,
							projInfo);
}

/*
 * ExecAssignScanProjectionInfo
 *		Set up projection info for a scan node, if necessary.
 *
 * We can avoid a projection step if the requested tlist exactly matches
 * the underlying tuple type.  If so, we just set ps_ProjInfo to NULL.
 * Note that this case occurs not only for simple "SELECT * FROM ...", but
 * also in most cases where there are joins or other processing nodes above
 * the scan node, because the planner will preferentially generate a matching
 * tlist.
 *
 * The scan slot's descriptor must have been set already.
 */
void
ExecAssignScanProjectionInfo(ScanState *node)
{
	Scan	   *scan = (Scan *) node->ps.plan;
	TupleDesc	tupdesc = node->ss_ScanTupleSlot->tts_tupleDescriptor;

	ExecConditionalAssignProjectionInfo(&node->ps, tupdesc, scan->scanrelid);
}

/*
 * ExecAssignScanProjectionInfoWithVarno
 *		As above, but caller can specify varno expected in Vars in the tlist.
 */
void
ExecAssignScanProjectionInfoWithVarno(ScanState *node, int varno)
{
	TupleDesc	tupdesc = node->ss_ScanTupleSlot->tts_tupleDescriptor;

	ExecConditionalAssignProjectionInfo(&node->ps, tupdesc, varno);
}

/*
 * ExecScanReScan
 *
 * This must be called within the ReScan function of any plan node type
 * that uses ExecScan().
 */
void
ExecScanReScan(ScanState *node)
{
	EState	   *estate = node->ps.state;

	/*
	 * We must clear the scan tuple so that observers (e.g., execCurrent.c)
	 * can tell that this plan node is not positioned on a tuple.
	 */
	ExecClearTuple(node->ss_ScanTupleSlot);

	/*
	 * Rescan EvalPlanQual tuple(s) if we're inside an EvalPlanQual recheck.
	 * But don't lose the "blocked" status of blocked target relations.
	 */
	if (estate->es_epq_active != NULL)
	{
		EPQState   *epqstate = estate->es_epq_active;
		Index		scanrelid = ((Scan *) node->ps.plan)->scanrelid;

		if (scanrelid > 0)
			epqstate->relsubs_done[scanrelid - 1] =
				epqstate->relsubs_blocked[scanrelid - 1];
		else
		{
			Bitmapset  *relids;
			int			rtindex = -1;

			/*
			 * If an FDW or custom scan provider has replaced the join with a
			 * scan, there are multiple RTIs; reset the epqScanDone flag for
			 * all of them.
			 */
			if (IsA(node->ps.plan, ForeignScan))
				relids = ((ForeignScan *) node->ps.plan)->fs_base_relids;
			else if (IsA(node->ps.plan, CustomScan))
				relids = ((CustomScan *) node->ps.plan)->custom_relids;
			else
				elog(ERROR, "unexpected scan node: %d",
					 (int) nodeTag(node->ps.plan));

			while ((rtindex = bms_next_member(relids, rtindex)) >= 0)
			{
				Assert(rtindex > 0);
				epqstate->relsubs_done[rtindex - 1] =
					epqstate->relsubs_blocked[rtindex - 1];
			}
		}
	}
}