postgres/src/backend/executor/nodeSeqscan.c

/*-------------------------------------------------------------------------
 *
 * nodeSeqscan.c
 *	  Support routines for sequential scans of relations.
 *
 * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *	  $PostgreSQL: pgsql/src/backend/executor/nodeSeqscan.c,v 1.52 2005/03/16 21:38:07 tgl Exp $
 *
 *-------------------------------------------------------------------------
 */
/*
 * INTERFACE ROUTINES
 *		ExecSeqScan				sequentially scans a relation.
 *		ExecSeqNext				retrieve next tuple in sequential order.
 *		ExecInitSeqScan			creates and initializes a seqscan node.
 *		ExecEndSeqScan			releases any storage allocated.
 *		ExecSeqReScan			rescans the relation
 *		ExecSeqMarkPos			marks scan position
 *		ExecSeqRestrPos			restores scan position
 */
#include "postgres.h"

#include "access/heapam.h"
#include "executor/execdebug.h"
#include "executor/nodeSeqscan.h"
#include "parser/parsetree.h"

static void InitScanRelation(SeqScanState *node, EState *estate);
static TupleTableSlot *SeqNext(SeqScanState *node);

/* ----------------------------------------------------------------
 *						Scan Support
 * ----------------------------------------------------------------
 */
/* ----------------------------------------------------------------
 *		SeqNext
 *
 *		This is a workhorse for ExecSeqScan
 * ----------------------------------------------------------------
 */
static TupleTableSlot *
SeqNext(SeqScanState *node)
{
	HeapTuple	tuple;
	HeapScanDesc scandesc;
	Index		scanrelid;
	EState	   *estate;
	ScanDirection direction;
	TupleTableSlot *slot;

	/*
	 * get information from the estate and scan state
	 */
	estate = node->ps.state;
	scandesc = node->ss_currentScanDesc;
	scanrelid = ((SeqScan *) node->ps.plan)->scanrelid;
	direction = estate->es_direction;
	slot = node->ss_ScanTupleSlot;

	/*
	 * Clear any reference to the previously returned tuple.  The idea
	 * here is to not have the tuple slot be the last holder of a pin on
	 * that tuple's buffer; if it is, we'll need a separate visit to the
	 * bufmgr to release the buffer.  By clearing here, we get to have the
	 * release done by ReleaseAndReadBuffer inside heap_getnext.
	 */
	ExecClearTuple(slot);

	/*
	 * Check if we are evaluating PlanQual for tuple of this relation.
	 * Additional checking is not good, but no other way for now. We could
	 * introduce new nodes for this case and handle SeqScan --> NewNode
	 * switching in Init/ReScan plan...
	 */
	if (estate->es_evTuple != NULL &&
		estate->es_evTuple[scanrelid - 1] != NULL)
	{
		if (estate->es_evTupleNull[scanrelid - 1])
			return slot;		/* return empty slot */

		ExecStoreTuple(estate->es_evTuple[scanrelid - 1],
					   slot, InvalidBuffer, false);

		/*
		 * Note that unlike IndexScan, SeqScan never use keys in
		 * heap_beginscan (and this is very bad) - so, here we do not
		 * check are keys ok or not.
		 */

		/* Flag for the next call that no more tuples */
		estate->es_evTupleNull[scanrelid - 1] = true;
		return (slot);
	}

	/*
	 * get the next tuple from the access methods
	 */
	tuple = heap_getnext(scandesc, direction);

	/*
	 * save the tuple and the buffer returned to us by the access methods
	 * in our scan tuple slot and return the slot.	Note: we pass 'false'
	 * because tuples returned by heap_getnext() are pointers onto disk
	 * pages and were not created with palloc() and so should not be
	 * pfree()'d.  Note also that ExecStoreTuple will increment the
	 * refcount of the buffer; the refcount will not be dropped until the
	 * tuple table slot is cleared.
	 */
	if (tuple)
		ExecStoreTuple(tuple,					/* tuple to store */
					   slot,					/* slot to store in */
					   scandesc->rs_cbuf,		/* buffer associated with
												 * this tuple */
					   false);		/* don't pfree this pointer */

	return slot;
}

/* ----------------------------------------------------------------
 *		ExecSeqScan(node)
 *
 *		Scans the relation sequentially and returns the next qualifying
 *		tuple.
 *		It calls the ExecScan() routine and passes it the access method
 *		which retrieve tuples sequentially.
 *
 */

TupleTableSlot *
ExecSeqScan(SeqScanState *node)
{
	/*
	 * use SeqNext as access method
	 */
	return ExecScan((ScanState *) node, (ExecScanAccessMtd) SeqNext);
}

/* ----------------------------------------------------------------
 *		InitScanRelation
 *
 *		This does the initialization for scan relations and
 *		subplans of scans.
 * ----------------------------------------------------------------
 */
static void
InitScanRelation(SeqScanState *node, EState *estate)
{
	Index		relid;
	List	   *rangeTable;
	RangeTblEntry *rtentry;
	Oid			reloid;
	Relation	currentRelation;
	HeapScanDesc currentScanDesc;

	/*
	 * get the relation object id from the relid'th entry in the range
	 * table, open that relation and initialize the scan state.
	 *
	 * We acquire AccessShareLock for the duration of the scan.
	 */
	relid = ((SeqScan *) node->ps.plan)->scanrelid;
	rangeTable = estate->es_range_table;
	rtentry = rt_fetch(relid, rangeTable);
	reloid = rtentry->relid;

	currentRelation = heap_open(reloid, AccessShareLock);

	currentScanDesc = heap_beginscan(currentRelation,
									 estate->es_snapshot,
									 0,
									 NULL);

	node->ss_currentRelation = currentRelation;
	node->ss_currentScanDesc = currentScanDesc;

	ExecAssignScanType(node, RelationGetDescr(currentRelation), false);
}


/* ----------------------------------------------------------------
 *		ExecInitSeqScan
 * ----------------------------------------------------------------
 */
SeqScanState *
ExecInitSeqScan(SeqScan *node, EState *estate)
{
	SeqScanState *scanstate;

	/*
	 * Once upon a time it was possible to have an outerPlan of a SeqScan,
	 * but not any more.
	 */
	Assert(outerPlan(node) == NULL);
	Assert(innerPlan(node) == NULL);

	/*
	 * create state structure
	 */
	scanstate = makeNode(SeqScanState);
	scanstate->ps.plan = (Plan *) node;
	scanstate->ps.state = estate;

	/*
	 * Miscellaneous initialization
	 *
	 * create expression context for node
	 */
	ExecAssignExprContext(estate, &scanstate->ps);

	/*
	 * initialize child expressions
	 */
	scanstate->ps.targetlist = (List *)
		ExecInitExpr((Expr *) node->plan.targetlist,
					 (PlanState *) scanstate);
	scanstate->ps.qual = (List *)
		ExecInitExpr((Expr *) node->plan.qual,
					 (PlanState *) scanstate);

#define SEQSCAN_NSLOTS 2

	/*
	 * tuple table initialization
	 */
	ExecInitResultTupleSlot(estate, &scanstate->ps);
	ExecInitScanTupleSlot(estate, scanstate);

	/*
	 * initialize scan relation
	 */
	InitScanRelation(scanstate, estate);

	scanstate->ps.ps_TupFromTlist = false;

	/*
	 * Initialize result tuple type and projection info.
	 */
	ExecAssignResultTypeFromTL(&scanstate->ps);
	ExecAssignScanProjectionInfo(scanstate);

	return scanstate;
}

int
ExecCountSlotsSeqScan(SeqScan *node)
{
	return ExecCountSlotsNode(outerPlan(node)) +
		ExecCountSlotsNode(innerPlan(node)) +
		SEQSCAN_NSLOTS;
}

/* ----------------------------------------------------------------
 *		ExecEndSeqScan
 *
 *		frees any storage allocated through C routines.
 * ----------------------------------------------------------------
 */
void
ExecEndSeqScan(SeqScanState *node)
{
	Relation	relation;
	HeapScanDesc scanDesc;

	/*
	 * get information from node
	 */
	relation = node->ss_currentRelation;
	scanDesc = node->ss_currentScanDesc;

	/*
	 * Free the exprcontext
	 */
	ExecFreeExprContext(&node->ps);

	/*
	 * clean out the tuple table
	 */
	ExecClearTuple(node->ps.ps_ResultTupleSlot);
	ExecClearTuple(node->ss_ScanTupleSlot);

	/*
	 * close heap scan
	 */
	heap_endscan(scanDesc);

	/*
	 * close the heap relation.
	 *
	 * Currently, we do not release the AccessShareLock acquired by
	 * InitScanRelation.  This lock should be held till end of
	 * transaction. (There is a faction that considers this too much
	 * locking, however.)
	 */
	heap_close(relation, NoLock);
}

/* ----------------------------------------------------------------
 *						Join Support
 * ----------------------------------------------------------------
 */

/* ----------------------------------------------------------------
 *		ExecSeqReScan
 *
 *		Rescans the relation.
 * ----------------------------------------------------------------
 */
void
ExecSeqReScan(SeqScanState *node, ExprContext *exprCtxt)
{
	EState	   *estate;
	Index		scanrelid;
	HeapScanDesc scan;

	estate = node->ps.state;
	scanrelid = ((SeqScan *) node->ps.plan)->scanrelid;

	/* If this is re-scanning of PlanQual ... */
	if (estate->es_evTuple != NULL &&
		estate->es_evTuple[scanrelid - 1] != NULL)
	{
		estate->es_evTupleNull[scanrelid - 1] = false;
		return;
	}

	scan = node->ss_currentScanDesc;

	heap_rescan(scan,			/* scan desc */
				NULL);			/* new scan keys */
}

/* ----------------------------------------------------------------
 *		ExecSeqMarkPos(node)
 *
 *		Marks scan position.
 * ----------------------------------------------------------------
 */
void
ExecSeqMarkPos(SeqScanState *node)
{
	HeapScanDesc scan;

	scan = node->ss_currentScanDesc;
	heap_markpos(scan);
}

/* ----------------------------------------------------------------
 *		ExecSeqRestrPos
 *
 *		Restores scan position.
 * ----------------------------------------------------------------
 */
void
ExecSeqRestrPos(SeqScanState *node)
{
	HeapScanDesc scan;

	scan = node->ss_currentScanDesc;
	heap_restrpos(scan);
}