granicus.if.org Git - postgresql/blob - src/backend/executor/execScan.c

   1 /*-------------------------------------------------------------------------
   2  *
   3  * execScan.c
   4  *        This code provides support for generalized relation scans. ExecScan
   5  *        is passed a node and a pointer to a function to "do the right thing"
   6  *        and return a tuple from the relation. ExecScan then does the tedious
   7  *        stuff - checking the qualification and projecting the tuple
   8  *        appropriately.
   9  *
  10  * Portions Copyright (c) 1996-2002, PostgreSQL Global Development Group
  11  * Portions Copyright (c) 1994, Regents of the University of California
  12  *
  13  *
  14  * IDENTIFICATION
  15  *        $Header: /cvsroot/pgsql/src/backend/executor/execScan.c,v 1.20 2002/06/20 20:29:27 momjian Exp $
  16  *
  17  *-------------------------------------------------------------------------
  18  */
  19 #include "postgres.h"
  20
  21 #include <sys/types.h>
  22 #include <sys/file.h>
  23
  24 #include "executor/executor.h"
  25 #include "miscadmin.h"
  26 #include "utils/memutils.h"
  27
  28
  29 /* ----------------------------------------------------------------
  30  *              ExecScan
  31  *
  32  *              Scans the relation using the 'access method' indicated and
  33  *              returns the next qualifying tuple in the direction specified
  34  *              in the global variable ExecDirection.
  35  *              The access method returns the next tuple and execScan() is
  36  *              responsible for checking the tuple returned against the qual-clause.
  37  *
  38  *              Conditions:
  39  *                -- the "cursor" maintained by the AMI is positioned at the tuple
  40  *                       returned previously.
  41  *
  42  *              Initial States:
  43  *                -- the relation indicated is opened for scanning so that the
  44  *                       "cursor" is positioned before the first qualifying tuple.
  45  * ----------------------------------------------------------------
  46  */
  47 TupleTableSlot *
  48 ExecScan(Scan *node,
  49                  ExecScanAccessMtd accessMtd)   /* function returning a tuple */
  50 {
  51         CommonScanState *scanstate;
  52         EState     *estate;
  53         ExprContext *econtext;
  54         List       *qual;
  55         ExprDoneCond isDone;
  56         TupleTableSlot *resultSlot;
  57
  58         /*
  59          * Fetch data from node
  60          */
  61         estate = node->plan.state;
  62         scanstate = node->scanstate;
  63         econtext = scanstate->cstate.cs_ExprContext;
  64         qual = node->plan.qual;
  65
  66         /*
  67          * Check to see if we're still projecting out tuples from a previous
  68          * scan tuple (because there is a function-returning-set in the
  69          * projection expressions).  If so, try to project another one.
  70          */
  71         if (scanstate->cstate.cs_TupFromTlist)
  72         {
  73                 resultSlot = ExecProject(scanstate->cstate.cs_ProjInfo, &isDone);
  74                 if (isDone == ExprMultipleResult)
  75                         return resultSlot;
  76                 /* Done with that source tuple... */
  77                 scanstate->cstate.cs_TupFromTlist = false;
  78         }
  79
  80         /*
  81          * Reset per-tuple memory context to free any expression evaluation
  82          * storage allocated in the previous tuple cycle.  Note this can't
  83          * happen until we're done projecting out tuples from a scan tuple.
  84          */
  85         ResetExprContext(econtext);
  86
  87         /*
  88          * get a tuple from the access method loop until we obtain a tuple
  89          * which passes the qualification.
  90          */
  91         for (;;)
  92         {
  93                 TupleTableSlot *slot;
  94
  95                 CHECK_FOR_INTERRUPTS();
  96
  97                 slot = (*accessMtd) (node);
  98
  99                 /*
 100                  * if the slot returned by the accessMtd contains NULL, then it
 101                  * means there is nothing more to scan so we just return an empty
 102                  * slot, being careful to use the projection result slot so it has
 103                  * correct tupleDesc.
 104                  */
 105                 if (TupIsNull(slot))
 106                 {
 107                         return ExecStoreTuple(NULL,
 108                                                                   scanstate->cstate.cs_ProjInfo->pi_slot,
 109                                                                   InvalidBuffer,
 110                                                                   true);
 111                 }
 112
 113                 /*
 114                  * place the current tuple into the expr context
 115                  */
 116                 econtext->ecxt_scantuple = slot;
 117
 118                 /*
 119                  * check that the current tuple satisfies the qual-clause
 120                  *
 121                  * check for non-nil qual here to avoid a function call to ExecQual()
 122                  * when the qual is nil ... saves only a few cycles, but they add
 123                  * up ...
 124                  */
 125                 if (!qual || ExecQual(qual, econtext, false))
 126                 {
 127                         /*
 128                          * Found a satisfactory scan tuple.
 129                          *
 130                          * Form a projection tuple, store it in the result tuple slot and
 131                          * return it --- unless we find we can project no tuples from
 132                          * this scan tuple, in which case continue scan.
 133                          */
 134                         resultSlot = ExecProject(scanstate->cstate.cs_ProjInfo, &isDone);
 135                         if (isDone != ExprEndResult)
 136                         {
 137                                 scanstate->cstate.cs_TupFromTlist = (isDone == ExprMultipleResult);
 138                                 return resultSlot;
 139                         }
 140                 }
 141
 142                 /*
 143                  * Tuple fails qual, so free per-tuple memory and try again.
 144                  */
 145                 ResetExprContext(econtext);
 146         }
 147 }