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252 lines
7.2 KiB
C
252 lines
7.2 KiB
C
/*-------------------------------------------------------------------------
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*
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* execScan.c
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* This code provides support for generalized relation scans. ExecScan
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* is passed a node and a pointer to a function to "do the right thing"
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* and return a tuple from the relation. ExecScan then does the tedious
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* stuff - checking the qualification and projecting the tuple
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* appropriately.
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*
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* Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
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* Portions Copyright (c) 1994, Regents of the University of California
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*
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*
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* IDENTIFICATION
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* $PostgreSQL: pgsql/src/backend/executor/execScan.c,v 1.46 2009/04/02 20:59:10 momjian Exp $
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*
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*-------------------------------------------------------------------------
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*/
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#include "postgres.h"
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#include "executor/executor.h"
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#include "miscadmin.h"
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#include "utils/memutils.h"
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static bool tlist_matches_tupdesc(PlanState *ps, List *tlist, Index varno, TupleDesc tupdesc);
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/* ----------------------------------------------------------------
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* ExecScan
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*
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* Scans the relation using the 'access method' indicated and
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* returns the next qualifying tuple in the direction specified
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* in the global variable ExecDirection.
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* The access method returns the next tuple and execScan() is
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* responsible for checking the tuple returned against the qual-clause.
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*
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* Conditions:
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* -- the "cursor" maintained by the AMI is positioned at the tuple
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* returned previously.
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*
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* Initial States:
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* -- the relation indicated is opened for scanning so that the
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* "cursor" is positioned before the first qualifying tuple.
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* ----------------------------------------------------------------
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*/
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TupleTableSlot *
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ExecScan(ScanState *node,
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ExecScanAccessMtd accessMtd) /* function returning a tuple */
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{
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ExprContext *econtext;
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List *qual;
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ProjectionInfo *projInfo;
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ExprDoneCond isDone;
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TupleTableSlot *resultSlot;
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/*
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* Fetch data from node
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*/
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qual = node->ps.qual;
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projInfo = node->ps.ps_ProjInfo;
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/*
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* If we have neither a qual to check nor a projection to do, just skip
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* all the overhead and return the raw scan tuple.
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*/
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if (!qual && !projInfo)
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return (*accessMtd) (node);
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/*
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* Check to see if we're still projecting out tuples from a previous scan
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* tuple (because there is a function-returning-set in the projection
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* expressions). If so, try to project another one.
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*/
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if (node->ps.ps_TupFromTlist)
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{
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Assert(projInfo); /* can't get here if not projecting */
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resultSlot = ExecProject(projInfo, &isDone);
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if (isDone == ExprMultipleResult)
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return resultSlot;
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/* Done with that source tuple... */
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node->ps.ps_TupFromTlist = false;
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}
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/*
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* Reset per-tuple memory context to free any expression evaluation
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* storage allocated in the previous tuple cycle. Note this can't happen
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* until we're done projecting out tuples from a scan tuple.
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*/
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econtext = node->ps.ps_ExprContext;
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ResetExprContext(econtext);
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/*
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* get a tuple from the access method loop until we obtain a tuple which
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* passes the qualification.
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*/
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for (;;)
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{
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TupleTableSlot *slot;
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CHECK_FOR_INTERRUPTS();
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slot = (*accessMtd) (node);
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/*
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* if the slot returned by the accessMtd contains NULL, then it means
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* there is nothing more to scan so we just return an empty slot,
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* being careful to use the projection result slot so it has correct
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* tupleDesc.
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*/
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if (TupIsNull(slot))
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{
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if (projInfo)
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return ExecClearTuple(projInfo->pi_slot);
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else
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return slot;
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}
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/*
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* place the current tuple into the expr context
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*/
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econtext->ecxt_scantuple = slot;
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/*
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* check that the current tuple satisfies the qual-clause
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*
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* check for non-nil qual here to avoid a function call to ExecQual()
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* when the qual is nil ... saves only a few cycles, but they add up
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* ...
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*/
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if (!qual || ExecQual(qual, econtext, false))
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{
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/*
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* Found a satisfactory scan tuple.
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*/
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if (projInfo)
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{
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/*
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* Form a projection tuple, store it in the result tuple slot
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* and return it --- unless we find we can project no tuples
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* from this scan tuple, in which case continue scan.
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*/
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resultSlot = ExecProject(projInfo, &isDone);
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if (isDone != ExprEndResult)
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{
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node->ps.ps_TupFromTlist = (isDone == ExprMultipleResult);
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return resultSlot;
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}
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}
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else
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{
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/*
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* Here, we aren't projecting, so just return scan tuple.
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*/
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return slot;
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}
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}
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/*
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* Tuple fails qual, so free per-tuple memory and try again.
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*/
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ResetExprContext(econtext);
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}
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}
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/*
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* ExecAssignScanProjectionInfo
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* Set up projection info for a scan node, if necessary.
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*
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* We can avoid a projection step if the requested tlist exactly matches
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* the underlying tuple type. If so, we just set ps_ProjInfo to NULL.
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* Note that this case occurs not only for simple "SELECT * FROM ...", but
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* also in most cases where there are joins or other processing nodes above
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* the scan node, because the planner will preferentially generate a matching
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* tlist.
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*
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* ExecAssignScanType must have been called already.
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*/
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void
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ExecAssignScanProjectionInfo(ScanState *node)
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{
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Scan *scan = (Scan *) node->ps.plan;
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if (tlist_matches_tupdesc(&node->ps,
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scan->plan.targetlist,
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scan->scanrelid,
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node->ss_ScanTupleSlot->tts_tupleDescriptor))
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node->ps.ps_ProjInfo = NULL;
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else
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ExecAssignProjectionInfo(&node->ps,
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node->ss_ScanTupleSlot->tts_tupleDescriptor);
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}
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static bool
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tlist_matches_tupdesc(PlanState *ps, List *tlist, Index varno, TupleDesc tupdesc)
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{
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int numattrs = tupdesc->natts;
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int attrno;
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bool hasoid;
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ListCell *tlist_item = list_head(tlist);
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/* Check the tlist attributes */
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for (attrno = 1; attrno <= numattrs; attrno++)
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{
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Form_pg_attribute att_tup = tupdesc->attrs[attrno - 1];
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Var *var;
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if (tlist_item == NULL)
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return false; /* tlist too short */
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var = (Var *) ((TargetEntry *) lfirst(tlist_item))->expr;
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if (!var || !IsA(var, Var))
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return false; /* tlist item not a Var */
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/* if these Asserts fail, planner messed up */
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Assert(var->varno == varno);
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Assert(var->varlevelsup == 0);
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if (var->varattno != attrno)
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return false; /* out of order */
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if (att_tup->attisdropped)
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return false; /* table contains dropped columns */
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/*
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* Note: usually the Var's type should match the tupdesc exactly, but
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* in situations involving unions of columns that have different
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* typmods, the Var may have come from above the union and hence have
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* typmod -1. This is a legitimate situation since the Var still
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* describes the column, just not as exactly as the tupdesc does. We
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* could change the planner to prevent it, but it'd then insert
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* projection steps just to convert from specific typmod to typmod -1,
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* which is pretty silly.
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*/
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if (var->vartype != att_tup->atttypid ||
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(var->vartypmod != att_tup->atttypmod &&
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var->vartypmod != -1))
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return false; /* type mismatch */
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tlist_item = lnext(tlist_item);
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}
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if (tlist_item)
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return false; /* tlist too long */
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/*
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* If the plan context requires a particular hasoid setting, then that has
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* to match, too.
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*/
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if (ExecContextForcesOids(ps, &hasoid) &&
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hasoid != tupdesc->tdhasoid)
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return false;
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return true;
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}
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