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) 1996-2004, PostgreSQL Global Development Group
  11  * Portions Copyright (c) 1994, Regents of the University of California
  12  *
  13  *
  14  * IDENTIFICATION
  15  *        $PostgreSQL: pgsql/src/backend/executor/execScan.c,v 1.32 2004/08/29 04:12:31 momjian Exp $
  16  *
  17  *-------------------------------------------------------------------------
  18  */
  19 #include "postgres.h"
  20
  21 #include "executor/executor.h"
  22 #include "miscadmin.h"
  23 #include "utils/memutils.h"
  24
  25
  26 static bool tlist_matches_tupdesc(PlanState *ps, List *tlist, Index varno, TupleDesc tupdesc);
  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(ScanState *node,
  49                  ExecScanAccessMtd accessMtd)   /* function returning a tuple */
  50 {
  51         EState     *estate;
  52         ExprContext *econtext;
  53         List       *qual;
  54         ProjectionInfo *projInfo;
  55         ExprDoneCond isDone;
  56         TupleTableSlot *resultSlot;
  57
  58         /*
  59          * Fetch data from node
  60          */
  61         estate = node->ps.state;
  62         econtext = node->ps.ps_ExprContext;
  63         qual = node->ps.qual;
  64         projInfo = node->ps.ps_ProjInfo;
  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 (node->ps.ps_TupFromTlist)
  72         {
  73                 Assert(projInfo);               /* can't get here if not projecting */
  74                 resultSlot = ExecProject(projInfo, &isDone);
  75                 if (isDone == ExprMultipleResult)
  76                         return resultSlot;
  77                 /* Done with that source tuple... */
  78                 node->ps.ps_TupFromTlist = false;
  79         }
  80
  81         /*
  82          * Reset per-tuple memory context to free any expression evaluation
  83          * storage allocated in the previous tuple cycle.  Note this can't
  84          * happen until we're done projecting out tuples from a scan tuple.
  85          */
  86         ResetExprContext(econtext);
  87
  88         /*
  89          * get a tuple from the access method loop until we obtain a tuple
  90          * which passes the qualification.
  91          */
  92         for (;;)
  93         {
  94                 TupleTableSlot *slot;
  95
  96                 CHECK_FOR_INTERRUPTS();
  97
  98                 slot = (*accessMtd) (node);
  99
 100                 /*
 101                  * if the slot returned by the accessMtd contains NULL, then it
 102                  * means there is nothing more to scan so we just return an empty
 103                  * slot, being careful to use the projection result slot so it has
 104                  * correct tupleDesc.
 105                  */
 106                 if (TupIsNull(slot))
 107                 {
 108                         if (projInfo)
 109                                 return ExecStoreTuple(NULL,
 110                                                                           projInfo->pi_slot,
 111                                                                           InvalidBuffer,
 112                                                                           true);
 113                         else
 114                                 return slot;
 115                 }
 116
 117                 /*
 118                  * place the current tuple into the expr context
 119                  */
 120                 econtext->ecxt_scantuple = slot;
 121
 122                 /*
 123                  * check that the current tuple satisfies the qual-clause
 124                  *
 125                  * check for non-nil qual here to avoid a function call to ExecQual()
 126                  * when the qual is nil ... saves only a few cycles, but they add
 127                  * up ...
 128                  */
 129                 if (!qual || ExecQual(qual, econtext, false))
 130                 {
 131                         /*
 132                          * Found a satisfactory scan tuple.
 133                          */
 134                         if (projInfo)
 135                         {
 136                                 /*
 137                                  * Form a projection tuple, store it in the result tuple
 138                                  * slot and return it --- unless we find we can project no
 139                                  * tuples from this scan tuple, in which case continue
 140                                  * scan.
 141                                  */
 142                                 resultSlot = ExecProject(projInfo, &isDone);
 143                                 if (isDone != ExprEndResult)
 144                                 {
 145                                         node->ps.ps_TupFromTlist = (isDone == ExprMultipleResult);
 146                                         return resultSlot;
 147                                 }
 148                         }
 149                         else
 150                         {
 151                                 /*
 152                                  * Here, we aren't projecting, so just return scan tuple.
 153                                  */
 154                                 return slot;
 155                         }
 156                 }
 157
 158                 /*
 159                  * Tuple fails qual, so free per-tuple memory and try again.
 160                  */
 161                 ResetExprContext(econtext);
 162         }
 163 }
 164
 165 /*
 166  * ExecAssignScanProjectionInfo
 167  *              Set up projection info for a scan node, if necessary.
 168  *
 169  * We can avoid a projection step if the requested tlist exactly matches
 170  * the underlying tuple type.  If so, we just set ps_ProjInfo to NULL.
 171  * Note that this case occurs not only for simple "SELECT * FROM ...", but
 172  * also in most cases where there are joins or other processing nodes above
 173  * the scan node, because the planner will preferentially generate a matching
 174  * tlist.
 175  *
 176  * ExecAssignScanType must have been called already.
 177  */
 178 void
 179 ExecAssignScanProjectionInfo(ScanState *node)
 180 {
 181         Scan       *scan = (Scan *) node->ps.plan;
 182
 183         if (tlist_matches_tupdesc(&node->ps,
 184                                                           scan->plan.targetlist,
 185                                                           scan->scanrelid,
 186                                                         node->ss_ScanTupleSlot->ttc_tupleDescriptor))
 187                 node->ps.ps_ProjInfo = NULL;
 188         else
 189                 ExecAssignProjectionInfo(&node->ps);
 190 }
 191
 192 static bool
 193 tlist_matches_tupdesc(PlanState *ps, List *tlist, Index varno, TupleDesc tupdesc)
 194 {
 195         int                     numattrs = tupdesc->natts;
 196         int                     attrno;
 197         bool            hasoid;
 198         ListCell   *tlist_item = list_head(tlist);
 199
 200         /* Check the tlist attributes */
 201         for (attrno = 1; attrno <= numattrs; attrno++)
 202         {
 203                 Form_pg_attribute att_tup = tupdesc->attrs[attrno - 1];
 204                 Var                *var;
 205
 206                 if (tlist_item == NULL)
 207                         return false;           /* tlist too short */
 208                 var = (Var *) ((TargetEntry *) lfirst(tlist_item))->expr;
 209                 if (!var || !IsA(var, Var))
 210                         return false;           /* tlist item not a Var */
 211                 Assert(var->varno == varno);
 212                 Assert(var->varlevelsup == 0);
 213                 if (var->varattno != attrno)
 214                         return false;           /* out of order */
 215                 if (att_tup->attisdropped)
 216                         return false;           /* table contains dropped columns */
 217                 Assert(var->vartype == att_tup->atttypid);
 218                 Assert(var->vartypmod == att_tup->atttypmod);
 219
 220                 tlist_item = lnext(tlist_item);
 221         }
 222
 223         if (tlist_item)
 224                 return false;                   /* tlist too long */
 225
 226         /*
 227          * If the plan context requires a particular hasoid setting, then
 228          * that has to match, too.
 229          */
 230         if (ExecContextForcesOids(ps, &hasoid) &&
 231                 hasoid != tupdesc->tdhasoid)
 232                 return false;
 233
 234         return true;
 235 }