1 /*-------------------------------------------------------------------------
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
10 * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
11 * Portions Copyright (c) 1994, Regents of the University of California
15 * $PostgreSQL: pgsql/src/backend/executor/execScan.c,v 1.40 2007/01/24 01:25:47 tgl Exp $
17 *-------------------------------------------------------------------------
21 #include "executor/executor.h"
22 #include "miscadmin.h"
23 #include "utils/memutils.h"
26 static bool tlist_matches_tupdesc(PlanState *ps, List *tlist, Index varno, TupleDesc tupdesc);
29 /* ----------------------------------------------------------------
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.
39 * -- the "cursor" maintained by the AMI is positioned at the tuple
40 * returned previously.
43 * -- the relation indicated is opened for scanning so that the
44 * "cursor" is positioned before the first qualifying tuple.
45 * ----------------------------------------------------------------
48 ExecScan(ScanState *node,
49 ExecScanAccessMtd accessMtd) /* function returning a tuple */
51 ExprContext *econtext;
53 ProjectionInfo *projInfo;
55 TupleTableSlot *resultSlot;
58 * Fetch data from node
61 projInfo = node->ps.ps_ProjInfo;
64 * If we have neither a qual to check nor a projection to do, just skip
65 * all the overhead and return the raw scan tuple.
67 if (!qual && !projInfo)
68 return (*accessMtd) (node);
71 * Check to see if we're still projecting out tuples from a previous scan
72 * tuple (because there is a function-returning-set in the projection
73 * expressions). If so, try to project another one.
75 if (node->ps.ps_TupFromTlist)
77 Assert(projInfo); /* can't get here if not projecting */
78 resultSlot = ExecProject(projInfo, &isDone);
79 if (isDone == ExprMultipleResult)
81 /* Done with that source tuple... */
82 node->ps.ps_TupFromTlist = false;
86 * Reset per-tuple memory context to free any expression evaluation
87 * storage allocated in the previous tuple cycle. Note this can't happen
88 * until we're done projecting out tuples from a scan tuple.
90 econtext = node->ps.ps_ExprContext;
91 ResetExprContext(econtext);
94 * get a tuple from the access method loop until we obtain a tuple which
95 * passes the qualification.
101 CHECK_FOR_INTERRUPTS();
103 slot = (*accessMtd) (node);
106 * if the slot returned by the accessMtd contains NULL, then it means
107 * there is nothing more to scan so we just return an empty slot,
108 * being careful to use the projection result slot so it has correct
114 return ExecClearTuple(projInfo->pi_slot);
120 * place the current tuple into the expr context
122 econtext->ecxt_scantuple = slot;
125 * check that the current tuple satisfies the qual-clause
127 * check for non-nil qual here to avoid a function call to ExecQual()
128 * when the qual is nil ... saves only a few cycles, but they add up
131 if (!qual || ExecQual(qual, econtext, false))
134 * Found a satisfactory scan tuple.
139 * Form a projection tuple, store it in the result tuple slot
140 * and return it --- unless we find we can project no tuples
141 * from this scan tuple, in which case continue scan.
143 resultSlot = ExecProject(projInfo, &isDone);
144 if (isDone != ExprEndResult)
146 node->ps.ps_TupFromTlist = (isDone == ExprMultipleResult);
153 * Here, we aren't projecting, so just return scan tuple.
160 * Tuple fails qual, so free per-tuple memory and try again.
162 ResetExprContext(econtext);
167 * ExecAssignScanProjectionInfo
168 * Set up projection info for a scan node, if necessary.
170 * We can avoid a projection step if the requested tlist exactly matches
171 * the underlying tuple type. If so, we just set ps_ProjInfo to NULL.
172 * Note that this case occurs not only for simple "SELECT * FROM ...", but
173 * also in most cases where there are joins or other processing nodes above
174 * the scan node, because the planner will preferentially generate a matching
177 * ExecAssignScanType must have been called already.
180 ExecAssignScanProjectionInfo(ScanState *node)
182 Scan *scan = (Scan *) node->ps.plan;
184 if (tlist_matches_tupdesc(&node->ps,
185 scan->plan.targetlist,
187 node->ss_ScanTupleSlot->tts_tupleDescriptor))
188 node->ps.ps_ProjInfo = NULL;
190 ExecAssignProjectionInfo(&node->ps);
194 tlist_matches_tupdesc(PlanState *ps, List *tlist, Index varno, TupleDesc tupdesc)
196 int numattrs = tupdesc->natts;
199 ListCell *tlist_item = list_head(tlist);
201 /* Check the tlist attributes */
202 for (attrno = 1; attrno <= numattrs; attrno++)
204 Form_pg_attribute att_tup = tupdesc->attrs[attrno - 1];
207 if (tlist_item == NULL)
208 return false; /* tlist too short */
209 var = (Var *) ((TargetEntry *) lfirst(tlist_item))->expr;
210 if (!var || !IsA(var, Var))
211 return false; /* tlist item not a Var */
212 Assert(var->varno == varno);
213 Assert(var->varlevelsup == 0);
214 if (var->varattno != attrno)
215 return false; /* out of order */
216 if (att_tup->attisdropped)
217 return false; /* table contains dropped columns */
219 * Note: usually the Var's type should match the tupdesc exactly,
220 * but in situations involving unions of columns that have different
221 * typmods, the Var may have come from above the union and hence have
222 * typmod -1. This is a legitimate situation since the Var still
223 * describes the column, just not as exactly as the tupdesc does.
224 * We could change the planner to prevent it, but it'd then insert
225 * projection steps just to convert from specific typmod to typmod -1,
226 * which is pretty silly.
228 Assert(var->vartype == att_tup->atttypid);
229 Assert(var->vartypmod == att_tup->atttypmod || var->vartypmod == -1);
231 tlist_item = lnext(tlist_item);
235 return false; /* tlist too long */
238 * If the plan context requires a particular hasoid setting, then that has
241 if (ExecContextForcesOids(ps, &hasoid) &&
242 hasoid != tupdesc->tdhasoid)