/*-------------------------------------------------------------------------
 *
 * 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) 1996-2003, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *	  $Header: /cvsroot/pgsql/src/backend/executor/execScan.c,v 1.26 2003/08/08 14:39:45 tgl Exp $
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "executor/executor.h"
#include "miscadmin.h"
#include "utils/memutils.h"


static bool tlist_matches_tupdesc(List *tlist, Index varno, TupleDesc tupdesc);


/* ----------------------------------------------------------------
 *		ExecScan
 *
 *		Scans the relation using the 'access method' indicated and
 *		returns the next qualifying tuple in the direction specified
 *		in the global variable ExecDirection.
 *		The access method returns the next tuple and execScan() is
 *		responsible for checking the tuple returned against the qual-clause.
 *
 *		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 */
{
	EState	   *estate;
	ExprContext *econtext;
	List	   *qual;
	ProjectionInfo *projInfo;
	ExprDoneCond isDone;
	TupleTableSlot *resultSlot;

	/*
	 * Fetch data from node
	 */
	estate = node->ps.state;
	econtext = node->ps.ps_ExprContext;
	qual = node->ps.qual;
	projInfo = node->ps.ps_ProjInfo;

	/*
	 * Check to see if we're still projecting out tuples from a previous
	 * scan tuple (because there is a function-returning-set in the
	 * projection expressions).  If so, try to project another one.
	 */
	if (node->ps.ps_TupFromTlist)
	{
		Assert(projInfo);		/* can't get here if not projecting */
		resultSlot = ExecProject(projInfo, &isDone);
		if (isDone == ExprMultipleResult)
			return resultSlot;
		/* Done with that source tuple... */
		node->ps.ps_TupFromTlist = false;
	}

	/*
	 * Reset per-tuple memory context to free any expression evaluation
	 * storage allocated in the previous tuple cycle.  Note this can't
	 * happen until we're done projecting out tuples from a scan tuple.
	 */
	ResetExprContext(econtext);

	/*
	 * get a tuple from the access method loop until we obtain a tuple
	 * which passes the qualification.
	 */
	for (;;)
	{
		TupleTableSlot *slot;

		CHECK_FOR_INTERRUPTS();

		slot = (*accessMtd) (node);

		/*
		 * if the slot returned by the accessMtd contains NULL, then it
		 * means there is nothing more to scan so we just return an empty
		 * slot, being careful to use the projection result slot so it has
		 * correct tupleDesc.
		 */
		if (TupIsNull(slot))
		{
			if (projInfo)
				return ExecStoreTuple(NULL,
									  projInfo->pi_slot,
									  InvalidBuffer,
									  true);
			else
				return slot;
		}

		/*
		 * place the current tuple into the expr context
		 */
		econtext->ecxt_scantuple = slot;

		/*
		 * check that the current tuple satisfies the qual-clause
		 *
		 * check for non-nil qual here to avoid a function call to ExecQual()
		 * when the qual is nil ... saves only a few cycles, but they add
		 * up ...
		 */
		if (!qual || ExecQual(qual, econtext, false))
		{
			/*
			 * Found a satisfactory scan tuple.
			 */
			if (projInfo)
			{
				/*
				 * Form a projection tuple, store it in the result tuple
				 * slot and return it --- unless we find we can project no
				 * tuples from this scan tuple, in which case continue
				 * scan.
				 */
				resultSlot = ExecProject(projInfo, &isDone);
				if (isDone != ExprEndResult)
				{
					node->ps.ps_TupFromTlist = (isDone == ExprMultipleResult);
					return resultSlot;
				}
			}
			else
			{
				/*
				 * Here, we aren't projecting, so just return scan tuple.
				 */
				return slot;
			}
		}

		/*
		 * Tuple fails qual, so free per-tuple memory and try again.
		 */
		ResetExprContext(econtext);
	}
}

/*
 * 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.
 *
 * ExecAssignScanType must have been called already.
 */
void
ExecAssignScanProjectionInfo(ScanState * node)
{
	Scan	   *scan = (Scan *) node->ps.plan;

	if (tlist_matches_tupdesc(scan->plan.targetlist,
							  scan->scanrelid,
							node->ss_ScanTupleSlot->ttc_tupleDescriptor))
		node->ps.ps_ProjInfo = NULL;
	else
		ExecAssignProjectionInfo(&node->ps);
}

static bool
tlist_matches_tupdesc(List *tlist, Index varno, TupleDesc tupdesc)
{
	int			numattrs = tupdesc->natts;
	int			attrno;

	for (attrno = 1; attrno <= numattrs; attrno++)
	{
#ifdef USE_ASSERT_CHECKING		/* only used in Assert() */
		Form_pg_attribute att_tup = tupdesc->attrs[attrno - 1];
#endif
		Var		   *var;

		if (tlist == NIL)
			return false;		/* tlist too short */
		var = (Var *) ((TargetEntry *) lfirst(tlist))->expr;
		if (!var || !IsA(var, Var))
			return false;		/* tlist item not a Var */
		Assert(var->varno == varno);
		if (var->varattno != attrno)
			return false;		/* out of order */
		Assert(var->vartype == att_tup->atttypid);
		Assert(var->vartypmod == att_tup->atttypmod);
		Assert(var->varlevelsup == 0);

		tlist = lnext(tlist);
	}

	if (tlist)
		return false;			/* tlist too long */

	return true;
}
