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) 1996-2005, 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 1.35 2005/03/16 21:38:06 tgl 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 ExecClearTuple(projInfo->pi_slot);
 110                         else
 111                                 return slot;
 112                 }
 113
 114                 /*
 115                  * place the current tuple into the expr context
 116                  */
 117                 econtext->ecxt_scantuple = slot;
 118
 119                 /*
 120                  * check that the current tuple satisfies the qual-clause
 121                  *
 122                  * check for non-nil qual here to avoid a function call to ExecQual()
 123                  * when the qual is nil ... saves only a few cycles, but they add
 124                  * up ...
 125                  */
 126                 if (!qual || ExecQual(qual, econtext, false))
 127                 {
 128                         /*
 129                          * Found a satisfactory scan tuple.
 130                          */
 131                         if (projInfo)
 132                         {
 133                                 /*
 134                                  * Form a projection tuple, store it in the result tuple
 135                                  * slot and return it --- unless we find we can project no
 136                                  * tuples from this scan tuple, in which case continue
 137                                  * scan.
 138                                  */
 139                                 resultSlot = ExecProject(projInfo, &isDone);
 140                                 if (isDone != ExprEndResult)
 141                                 {
 142                                         node->ps.ps_TupFromTlist = (isDone == ExprMultipleResult);
 143                                         return resultSlot;
 144                                 }
 145                         }
 146                         else
 147                         {
 148                                 /*
 149                                  * Here, we aren't projecting, so just return scan tuple.
 150                                  */
 151                                 return slot;
 152                         }
 153                 }
 154
 155                 /*
 156                  * Tuple fails qual, so free per-tuple memory and try again.
 157                  */
 158                 ResetExprContext(econtext);
 159         }
 160 }
 161
 162 /*
 163  * ExecAssignScanProjectionInfo
 164  *              Set up projection info for a scan node, if necessary.
 165  *
 166  * We can avoid a projection step if the requested tlist exactly matches
 167  * the underlying tuple type.  If so, we just set ps_ProjInfo to NULL.
 168  * Note that this case occurs not only for simple "SELECT * FROM ...", but
 169  * also in most cases where there are joins or other processing nodes above
 170  * the scan node, because the planner will preferentially generate a matching
 171  * tlist.
 172  *
 173  * ExecAssignScanType must have been called already.
 174  */
 175 void
 176 ExecAssignScanProjectionInfo(ScanState *node)
 177 {
 178         Scan       *scan = (Scan *) node->ps.plan;
 179
 180         if (tlist_matches_tupdesc(&node->ps,
 181                                                           scan->plan.targetlist,
 182                                                           scan->scanrelid,
 183                                                           node->ss_ScanTupleSlot->tts_tupleDescriptor))
 184                 node->ps.ps_ProjInfo = NULL;
 185         else
 186                 ExecAssignProjectionInfo(&node->ps);
 187 }
 188
 189 static bool
 190 tlist_matches_tupdesc(PlanState *ps, List *tlist, Index varno, TupleDesc tupdesc)
 191 {
 192         int                     numattrs = tupdesc->natts;
 193         int                     attrno;
 194         bool            hasoid;
 195         ListCell   *tlist_item = list_head(tlist);
 196
 197         /* Check the tlist attributes */
 198         for (attrno = 1; attrno <= numattrs; attrno++)
 199         {
 200                 Form_pg_attribute att_tup = tupdesc->attrs[attrno - 1];
 201                 Var                *var;
 202
 203                 if (tlist_item == NULL)
 204                         return false;           /* tlist too short */
 205                 var = (Var *) ((TargetEntry *) lfirst(tlist_item))->expr;
 206                 if (!var || !IsA(var, Var))
 207                         return false;           /* tlist item not a Var */
 208                 Assert(var->varno == varno);
 209                 Assert(var->varlevelsup == 0);
 210                 if (var->varattno != attrno)
 211                         return false;           /* out of order */
 212                 if (att_tup->attisdropped)
 213                         return false;           /* table contains dropped columns */
 214                 Assert(var->vartype == att_tup->atttypid);
 215                 Assert(var->vartypmod == att_tup->atttypmod);
 216
 217                 tlist_item = lnext(tlist_item);
 218         }
 219
 220         if (tlist_item)
 221                 return false;                   /* tlist too long */
 222
 223         /*
 224          * If the plan context requires a particular hasoid setting, then that
 225          * has to match, too.
 226          */
 227         if (ExecContextForcesOids(ps, &hasoid) &&
 228                 hasoid != tupdesc->tdhasoid)
 229                 return false;
 230
 231         return true;
 232 }