src/backend/executor/nodeResult.c

   1 /*-------------------------------------------------------------------------
   2  *
   3  * nodeResult.c
   4  *        support for constant nodes needing special code.
   5  *
   6  * DESCRIPTION
   7  *
   8  *              Result nodes are used in queries where no relations are scanned.
   9  *              Examples of such queries are:
  10  *
  11  *                              select 1 * 2
  12  *
  13  *                              insert into emp values ('mike', 15000)
  14  *
  15  *              (Remember that in an INSERT or UPDATE, we need a plan tree that
  16  *              generates the new rows.)
  17  *
  18  *              Result nodes are also used to optimise queries with constant
  19  *              qualifications (ie, quals that do not depend on the scanned data),
  20  *              such as:
  21  *
  22  *                              select * from emp where 2 > 1
  23  *
  24  *              In this case, the plan generated is
  25  *
  26  *                                              Result  (with 2 > 1 qual)
  27  *                                              /
  28  *                                 SeqScan (emp.*)
  29  *
  30  *              At runtime, the Result node evaluates the constant qual once,
  31  *              which is shown by EXPLAIN as a One-Time Filter.  If it's
  32  *              false, we can return an empty result set without running the
  33  *              controlled plan at all.  If it's true, we run the controlled
  34  *              plan normally and pass back the results.
  35  *
  36  *
  37  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  38  * Portions Copyright (c) 1994, Regents of the University of California
  39  *
  40  * IDENTIFICATION
  41  *        $PostgreSQL$
  42  *
  43  *-------------------------------------------------------------------------
  44  */
  45
  46 #include "postgres.h"
  47
  48 #include "executor/executor.h"
  49 #include "executor/nodeResult.h"
  50 #include "utils/memutils.h"
  51
  52
  53 /* ----------------------------------------------------------------
  54  *              ExecResult(node)
  55  *
  56  *              returns the tuples from the outer plan which satisfy the
  57  *              qualification clause.  Since result nodes with right
  58  *              subtrees are never planned, we ignore the right subtree
  59  *              entirely (for now).. -cim 10/7/89
  60  *
  61  *              The qualification containing only constant clauses are
  62  *              checked first before any processing is done. It always returns
  63  *              'nil' if the constant qualification is not satisfied.
  64  * ----------------------------------------------------------------
  65  */
  66 TupleTableSlot *
  67 ExecResult(ResultState *node)
  68 {
  69         TupleTableSlot *outerTupleSlot;
  70         TupleTableSlot *resultSlot;
  71         PlanState  *outerPlan;
  72         ExprContext *econtext;
  73         ExprDoneCond isDone;
  74
  75         econtext = node->ps.ps_ExprContext;
  76
  77         /*
  78          * check constant qualifications like (2 > 1), if not already done
  79          */
  80         if (node->rs_checkqual)
  81         {
  82                 bool            qualResult = ExecQual((List *) node->resconstantqual,
  83                                                                                   econtext,
  84                                                                                   false);
  85
  86                 node->rs_checkqual = false;
  87                 if (!qualResult)
  88                 {
  89                         node->rs_done = true;
  90                         return NULL;
  91                 }
  92         }
  93
  94         /*
  95          * Check to see if we're still projecting out tuples from a previous scan
  96          * tuple (because there is a function-returning-set in the projection
  97          * expressions).  If so, try to project another one.
  98          */
  99         if (node->ps.ps_TupFromTlist)
 100         {
 101                 resultSlot = ExecProject(node->ps.ps_ProjInfo, &isDone);
 102                 if (isDone == ExprMultipleResult)
 103                         return resultSlot;
 104                 /* Done with that source tuple... */
 105                 node->ps.ps_TupFromTlist = false;
 106         }
 107
 108         /*
 109          * Reset per-tuple memory context to free any expression evaluation
 110          * storage allocated in the previous tuple cycle.  Note this can't happen
 111          * until we're done projecting out tuples from a scan tuple.
 112          */
 113         ResetExprContext(econtext);
 114
 115         /*
 116          * if rs_done is true then it means that we were asked to return a
 117          * constant tuple and we already did the last time ExecResult() was
 118          * called, OR that we failed the constant qual check. Either way, now we
 119          * are through.
 120          */
 121         while (!node->rs_done)
 122         {
 123                 outerPlan = outerPlanState(node);
 124
 125                 if (outerPlan != NULL)
 126                 {
 127                         /*
 128                          * retrieve tuples from the outer plan until there are no more.
 129                          */
 130                         outerTupleSlot = ExecProcNode(outerPlan);
 131
 132                         if (TupIsNull(outerTupleSlot))
 133                                 return NULL;
 134
 135                         /*
 136                          * prepare to compute projection expressions, which will expect to
 137                          * access the input tuples as varno OUTER.
 138                          */
 139                         econtext->ecxt_outertuple = outerTupleSlot;
 140                 }
 141                 else
 142                 {
 143                         /*
 144                          * if we don't have an outer plan, then we are just generating the
 145                          * results from a constant target list.  Do it only once.
 146                          */
 147                         node->rs_done = true;
 148                 }
 149
 150                 /*
 151                  * form the result tuple using ExecProject(), and return it --- unless
 152                  * the projection produces an empty set, in which case we must loop
 153                  * back to see if there are more outerPlan tuples.
 154                  */
 155                 resultSlot = ExecProject(node->ps.ps_ProjInfo, &isDone);
 156
 157                 if (isDone != ExprEndResult)
 158                 {
 159                         node->ps.ps_TupFromTlist = (isDone == ExprMultipleResult);
 160                         return resultSlot;
 161                 }
 162         }
 163
 164         return NULL;
 165 }
 166
 167 /* ----------------------------------------------------------------
 168  *              ExecResultMarkPos
 169  * ----------------------------------------------------------------
 170  */
 171 void
 172 ExecResultMarkPos(ResultState *node)
 173 {
 174         PlanState  *outerPlan = outerPlanState(node);
 175
 176         if (outerPlan != NULL)
 177                 ExecMarkPos(outerPlan);
 178         else
 179                 elog(DEBUG2, "Result nodes do not support mark/restore");
 180 }
 181
 182 /* ----------------------------------------------------------------
 183  *              ExecResultRestrPos
 184  * ----------------------------------------------------------------
 185  */
 186 void
 187 ExecResultRestrPos(ResultState *node)
 188 {
 189         PlanState  *outerPlan = outerPlanState(node);
 190
 191         if (outerPlan != NULL)
 192                 ExecRestrPos(outerPlan);
 193         else
 194                 elog(ERROR, "Result nodes do not support mark/restore");
 195 }
 196
 197 /* ----------------------------------------------------------------
 198  *              ExecInitResult
 199  *
 200  *              Creates the run-time state information for the result node
 201  *              produced by the planner and initializes outer relations
 202  *              (child nodes).
 203  * ----------------------------------------------------------------
 204  */
 205 ResultState *
 206 ExecInitResult(Result *node, EState *estate, int eflags)
 207 {
 208         ResultState *resstate;
 209
 210         /* check for unsupported flags */
 211         Assert(!(eflags & (EXEC_FLAG_MARK | EXEC_FLAG_BACKWARD)) ||
 212                    outerPlan(node) != NULL);
 213
 214         /*
 215          * create state structure
 216          */
 217         resstate = makeNode(ResultState);
 218         resstate->ps.plan = (Plan *) node;
 219         resstate->ps.state = estate;
 220
 221         resstate->rs_done = false;
 222         resstate->rs_checkqual = (node->resconstantqual == NULL) ? false : true;
 223
 224         /*
 225          * Miscellaneous initialization
 226          *
 227          * create expression context for node
 228          */
 229         ExecAssignExprContext(estate, &resstate->ps);
 230
 231         resstate->ps.ps_TupFromTlist = false;
 232
 233 #define RESULT_NSLOTS 1
 234
 235         /*
 236          * tuple table initialization
 237          */
 238         ExecInitResultTupleSlot(estate, &resstate->ps);
 239
 240         /*
 241          * initialize child expressions
 242          */
 243         resstate->ps.targetlist = (List *)
 244                 ExecInitExpr((Expr *) node->plan.targetlist,
 245                                          (PlanState *) resstate);
 246         resstate->ps.qual = (List *)
 247                 ExecInitExpr((Expr *) node->plan.qual,
 248                                          (PlanState *) resstate);
 249         resstate->resconstantqual = ExecInitExpr((Expr *) node->resconstantqual,
 250                                                                                          (PlanState *) resstate);
 251
 252         /*
 253          * initialize child nodes
 254          */
 255         outerPlanState(resstate) = ExecInitNode(outerPlan(node), estate, eflags);
 256
 257         /*
 258          * we don't use inner plan
 259          */
 260         Assert(innerPlan(node) == NULL);
 261
 262         /*
 263          * initialize tuple type and projection info
 264          */
 265         ExecAssignResultTypeFromTL(&resstate->ps);
 266         ExecAssignProjectionInfo(&resstate->ps, NULL);
 267
 268         return resstate;
 269 }
 270
 271 int
 272 ExecCountSlotsResult(Result *node)
 273 {
 274         return ExecCountSlotsNode(outerPlan(node)) + RESULT_NSLOTS;
 275 }
 276
 277 /* ----------------------------------------------------------------
 278  *              ExecEndResult
 279  *
 280  *              frees up storage allocated through C routines
 281  * ----------------------------------------------------------------
 282  */
 283 void
 284 ExecEndResult(ResultState *node)
 285 {
 286         /*
 287          * Free the exprcontext
 288          */
 289         ExecFreeExprContext(&node->ps);
 290
 291         /*
 292          * clean out the tuple table
 293          */
 294         ExecClearTuple(node->ps.ps_ResultTupleSlot);
 295
 296         /*
 297          * shut down subplans
 298          */
 299         ExecEndNode(outerPlanState(node));
 300 }
 301
 302 void
 303 ExecReScanResult(ResultState *node, ExprContext *exprCtxt)
 304 {
 305         node->rs_done = false;
 306         node->ps.ps_TupFromTlist = false;
 307         node->rs_checkqual = (node->resconstantqual == NULL) ? false : true;
 308
 309         /*
 310          * If chgParam of subnode is not null then plan will be re-scanned by
 311          * first ExecProcNode.  However, if caller is passing us an exprCtxt then
 312          * forcibly rescan the subnode now, so that we can pass the exprCtxt down
 313          * to the subnode (needed for gated indexscan).
 314          */
 315         if (node->ps.lefttree &&
 316                 (node->ps.lefttree->chgParam == NULL || exprCtxt != NULL))
 317                 ExecReScan(node->ps.lefttree, exprCtxt);
 318 }