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nodeSamplescan.c
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1 /*-------------------------------------------------------------------------
2  *
3  * nodeSamplescan.c
4  * Support routines for sample scans of relations (table sampling).
5  *
6  * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
7  * Portions Copyright (c) 1994, Regents of the University of California
8  *
9  *
10  * IDENTIFICATION
11  * src/backend/executor/nodeSamplescan.c
12  *
13  *-------------------------------------------------------------------------
14  */
15 #include "postgres.h"
16 
17 #include "access/hash.h"
18 #include "access/relscan.h"
19 #include "access/tsmapi.h"
20 #include "executor/executor.h"
22 #include "miscadmin.h"
23 #include "pgstat.h"
24 #include "storage/predicate.h"
25 #include "utils/builtins.h"
26 #include "utils/rel.h"
27 #include "utils/tqual.h"
28 
29 static void InitScanRelation(SampleScanState *node, EState *estate, int eflags);
31 static void tablesample_init(SampleScanState *scanstate);
33 static bool SampleTupleVisible(HeapTuple tuple, OffsetNumber tupoffset,
34  HeapScanDesc scan);
35 
36 /* ----------------------------------------------------------------
37  * Scan Support
38  * ----------------------------------------------------------------
39  */
40 
41 /* ----------------------------------------------------------------
42  * SampleNext
43  *
44  * This is a workhorse for ExecSampleScan
45  * ----------------------------------------------------------------
46  */
47 static TupleTableSlot *
49 {
50  HeapTuple tuple;
51  TupleTableSlot *slot;
52 
53  /*
54  * if this is first call within a scan, initialize
55  */
56  if (!node->begun)
57  tablesample_init(node);
58 
59  /*
60  * get the next tuple, and store it in our result slot
61  */
62  tuple = tablesample_getnext(node);
63 
64  slot = node->ss.ss_ScanTupleSlot;
65 
66  if (tuple)
67  ExecStoreTuple(tuple, /* tuple to store */
68  slot, /* slot to store in */
69  node->ss.ss_currentScanDesc->rs_cbuf, /* tuple's buffer */
70  false); /* don't pfree this pointer */
71  else
72  ExecClearTuple(slot);
73 
74  return slot;
75 }
76 
77 /*
78  * SampleRecheck -- access method routine to recheck a tuple in EvalPlanQual
79  */
80 static bool
82 {
83  /*
84  * No need to recheck for SampleScan, since like SeqScan we don't pass any
85  * checkable keys to heap_beginscan.
86  */
87  return true;
88 }
89 
90 /* ----------------------------------------------------------------
91  * ExecSampleScan(node)
92  *
93  * Scans the relation using the sampling method and returns
94  * the next qualifying tuple.
95  * We call the ExecScan() routine and pass it the appropriate
96  * access method functions.
97  * ----------------------------------------------------------------
98  */
101 {
102  return ExecScan((ScanState *) node,
105 }
106 
107 /* ----------------------------------------------------------------
108  * InitScanRelation
109  *
110  * Set up to access the scan relation.
111  * ----------------------------------------------------------------
112  */
113 static void
114 InitScanRelation(SampleScanState *node, EState *estate, int eflags)
115 {
116  Relation currentRelation;
117 
118  /*
119  * get the relation object id from the relid'th entry in the range table,
120  * open that relation and acquire appropriate lock on it.
121  */
122  currentRelation = ExecOpenScanRelation(estate,
123  ((SampleScan *) node->ss.ps.plan)->scan.scanrelid,
124  eflags);
125 
126  node->ss.ss_currentRelation = currentRelation;
127 
128  /* we won't set up the HeapScanDesc till later */
129  node->ss.ss_currentScanDesc = NULL;
130 
131  /* and report the scan tuple slot's rowtype */
132  ExecAssignScanType(&node->ss, RelationGetDescr(currentRelation));
133 }
134 
135 
136 /* ----------------------------------------------------------------
137  * ExecInitSampleScan
138  * ----------------------------------------------------------------
139  */
141 ExecInitSampleScan(SampleScan *node, EState *estate, int eflags)
142 {
143  SampleScanState *scanstate;
144  TableSampleClause *tsc = node->tablesample;
145  TsmRoutine *tsm;
146 
147  Assert(outerPlan(node) == NULL);
148  Assert(innerPlan(node) == NULL);
149 
150  /*
151  * create state structure
152  */
153  scanstate = makeNode(SampleScanState);
154  scanstate->ss.ps.plan = (Plan *) node;
155  scanstate->ss.ps.state = estate;
156 
157  /*
158  * Miscellaneous initialization
159  *
160  * create expression context for node
161  */
162  ExecAssignExprContext(estate, &scanstate->ss.ps);
163 
164  /*
165  * initialize child expressions
166  */
167  scanstate->ss.ps.targetlist = (List *)
169  (PlanState *) scanstate);
170  scanstate->ss.ps.qual = (List *)
171  ExecInitExpr((Expr *) node->scan.plan.qual,
172  (PlanState *) scanstate);
173 
174  scanstate->args = (List *)
175  ExecInitExpr((Expr *) tsc->args,
176  (PlanState *) scanstate);
177  scanstate->repeatable =
179  (PlanState *) scanstate);
180 
181  /*
182  * tuple table initialization
183  */
184  ExecInitResultTupleSlot(estate, &scanstate->ss.ps);
185  ExecInitScanTupleSlot(estate, &scanstate->ss);
186 
187  /*
188  * initialize scan relation
189  */
190  InitScanRelation(scanstate, estate, eflags);
191 
192  /*
193  * Initialize result tuple type and projection info.
194  */
195  ExecAssignResultTypeFromTL(&scanstate->ss.ps);
196  ExecAssignScanProjectionInfo(&scanstate->ss);
197 
198  /*
199  * If we don't have a REPEATABLE clause, select a random seed. We want to
200  * do this just once, since the seed shouldn't change over rescans.
201  */
202  if (tsc->repeatable == NULL)
203  scanstate->seed = random();
204 
205  /*
206  * Finally, initialize the TABLESAMPLE method handler.
207  */
208  tsm = GetTsmRoutine(tsc->tsmhandler);
209  scanstate->tsmroutine = tsm;
210  scanstate->tsm_state = NULL;
211 
212  if (tsm->InitSampleScan)
213  tsm->InitSampleScan(scanstate, eflags);
214 
215  /* We'll do BeginSampleScan later; we can't evaluate params yet */
216  scanstate->begun = false;
217 
218  return scanstate;
219 }
220 
221 /* ----------------------------------------------------------------
222  * ExecEndSampleScan
223  *
224  * frees any storage allocated through C routines.
225  * ----------------------------------------------------------------
226  */
227 void
229 {
230  /*
231  * Tell sampling function that we finished the scan.
232  */
233  if (node->tsmroutine->EndSampleScan)
234  node->tsmroutine->EndSampleScan(node);
235 
236  /*
237  * Free the exprcontext
238  */
239  ExecFreeExprContext(&node->ss.ps);
240 
241  /*
242  * clean out the tuple table
243  */
246 
247  /*
248  * close heap scan
249  */
250  if (node->ss.ss_currentScanDesc)
252 
253  /*
254  * close the heap relation.
255  */
257 }
258 
259 /* ----------------------------------------------------------------
260  * ExecReScanSampleScan
261  *
262  * Rescans the relation.
263  *
264  * ----------------------------------------------------------------
265  */
266 void
268 {
269  /* Remember we need to do BeginSampleScan again (if we did it at all) */
270  node->begun = false;
271 
272  ExecScanReScan(&node->ss);
273 }
274 
275 
276 /*
277  * Initialize the TABLESAMPLE method: evaluate params and call BeginSampleScan.
278  */
279 static void
281 {
282  TsmRoutine *tsm = scanstate->tsmroutine;
283  ExprContext *econtext = scanstate->ss.ps.ps_ExprContext;
284  Datum *params;
285  Datum datum;
286  bool isnull;
287  uint32 seed;
288  bool allow_sync;
289  int i;
290  ListCell *arg;
291 
292  params = (Datum *) palloc(list_length(scanstate->args) * sizeof(Datum));
293 
294  i = 0;
295  foreach(arg, scanstate->args)
296  {
297  ExprState *argstate = (ExprState *) lfirst(arg);
298 
299  params[i] = ExecEvalExprSwitchContext(argstate,
300  econtext,
301  &isnull);
302  if (isnull)
303  ereport(ERROR,
304  (errcode(ERRCODE_INVALID_TABLESAMPLE_ARGUMENT),
305  errmsg("TABLESAMPLE parameter cannot be null")));
306  i++;
307  }
308 
309  if (scanstate->repeatable)
310  {
311  datum = ExecEvalExprSwitchContext(scanstate->repeatable,
312  econtext,
313  &isnull);
314  if (isnull)
315  ereport(ERROR,
316  (errcode(ERRCODE_INVALID_TABLESAMPLE_REPEAT),
317  errmsg("TABLESAMPLE REPEATABLE parameter cannot be null")));
318 
319  /*
320  * The REPEATABLE parameter has been coerced to float8 by the parser.
321  * The reason for using float8 at the SQL level is that it will
322  * produce unsurprising results both for users used to databases that
323  * accept only integers in the REPEATABLE clause and for those who
324  * might expect that REPEATABLE works like setseed() (a float in the
325  * range from -1 to 1).
326  *
327  * We use hashfloat8() to convert the supplied value into a suitable
328  * seed. For regression-testing purposes, that has the convenient
329  * property that REPEATABLE(0) gives a machine-independent result.
330  */
332  }
333  else
334  {
335  /* Use the seed selected by ExecInitSampleScan */
336  seed = scanstate->seed;
337  }
338 
339  /* Set default values for params that BeginSampleScan can adjust */
340  scanstate->use_bulkread = true;
341  scanstate->use_pagemode = true;
342 
343  /* Let tablesample method do its thing */
344  tsm->BeginSampleScan(scanstate,
345  params,
346  list_length(scanstate->args),
347  seed);
348 
349  /* We'll use syncscan if there's no NextSampleBlock function */
350  allow_sync = (tsm->NextSampleBlock == NULL);
351 
352  /* Now we can create or reset the HeapScanDesc */
353  if (scanstate->ss.ss_currentScanDesc == NULL)
354  {
355  scanstate->ss.ss_currentScanDesc =
357  scanstate->ss.ps.state->es_snapshot,
358  0, NULL,
359  scanstate->use_bulkread,
360  allow_sync,
361  scanstate->use_pagemode);
362  }
363  else
364  {
366  scanstate->use_bulkread,
367  allow_sync,
368  scanstate->use_pagemode);
369  }
370 
371  pfree(params);
372 
373  /* And we're initialized. */
374  scanstate->begun = true;
375 }
376 
377 /*
378  * Get next tuple from TABLESAMPLE method.
379  *
380  * Note: an awful lot of this is copied-and-pasted from heapam.c. It would
381  * perhaps be better to refactor to share more code.
382  */
383 static HeapTuple
385 {
386  TsmRoutine *tsm = scanstate->tsmroutine;
387  HeapScanDesc scan = scanstate->ss.ss_currentScanDesc;
388  HeapTuple tuple = &(scan->rs_ctup);
389  Snapshot snapshot = scan->rs_snapshot;
390  bool pagemode = scan->rs_pageatatime;
391  BlockNumber blockno;
392  Page page;
393  bool all_visible;
394  OffsetNumber maxoffset;
395 
396  if (!scan->rs_inited)
397  {
398  /*
399  * return null immediately if relation is empty
400  */
401  if (scan->rs_nblocks == 0)
402  {
403  Assert(!BufferIsValid(scan->rs_cbuf));
404  tuple->t_data = NULL;
405  return NULL;
406  }
407  if (tsm->NextSampleBlock)
408  {
409  blockno = tsm->NextSampleBlock(scanstate);
410  if (!BlockNumberIsValid(blockno))
411  {
412  tuple->t_data = NULL;
413  return NULL;
414  }
415  }
416  else
417  blockno = scan->rs_startblock;
418  Assert(blockno < scan->rs_nblocks);
419  heapgetpage(scan, blockno);
420  scan->rs_inited = true;
421  }
422  else
423  {
424  /* continue from previously returned page/tuple */
425  blockno = scan->rs_cblock; /* current page */
426  }
427 
428  /*
429  * When not using pagemode, we must lock the buffer during tuple
430  * visibility checks.
431  */
432  if (!pagemode)
434 
435  page = (Page) BufferGetPage(scan->rs_cbuf);
436  all_visible = PageIsAllVisible(page) && !snapshot->takenDuringRecovery;
437  maxoffset = PageGetMaxOffsetNumber(page);
438 
439  for (;;)
440  {
441  OffsetNumber tupoffset;
442  bool finished;
443 
445 
446  /* Ask the tablesample method which tuples to check on this page. */
447  tupoffset = tsm->NextSampleTuple(scanstate,
448  blockno,
449  maxoffset);
450 
451  if (OffsetNumberIsValid(tupoffset))
452  {
453  ItemId itemid;
454  bool visible;
455 
456  /* Skip invalid tuple pointers. */
457  itemid = PageGetItemId(page, tupoffset);
458  if (!ItemIdIsNormal(itemid))
459  continue;
460 
461  tuple->t_data = (HeapTupleHeader) PageGetItem(page, itemid);
462  tuple->t_len = ItemIdGetLength(itemid);
463  ItemPointerSet(&(tuple->t_self), blockno, tupoffset);
464 
465  if (all_visible)
466  visible = true;
467  else
468  visible = SampleTupleVisible(tuple, tupoffset, scan);
469 
470  /* in pagemode, heapgetpage did this for us */
471  if (!pagemode)
472  CheckForSerializableConflictOut(visible, scan->rs_rd, tuple,
473  scan->rs_cbuf, snapshot);
474 
475  if (visible)
476  {
477  /* Found visible tuple, return it. */
478  if (!pagemode)
480  break;
481  }
482  else
483  {
484  /* Try next tuple from same page. */
485  continue;
486  }
487  }
488 
489  /*
490  * if we get here, it means we've exhausted the items on this page and
491  * it's time to move to the next.
492  */
493  if (!pagemode)
495 
496  if (tsm->NextSampleBlock)
497  {
498  blockno = tsm->NextSampleBlock(scanstate);
499  Assert(!scan->rs_syncscan);
500  finished = !BlockNumberIsValid(blockno);
501  }
502  else
503  {
504  /* Without NextSampleBlock, just do a plain forward seqscan. */
505  blockno++;
506  if (blockno >= scan->rs_nblocks)
507  blockno = 0;
508 
509  /*
510  * Report our new scan position for synchronization purposes.
511  *
512  * Note: we do this before checking for end of scan so that the
513  * final state of the position hint is back at the start of the
514  * rel. That's not strictly necessary, but otherwise when you run
515  * the same query multiple times the starting position would shift
516  * a little bit backwards on every invocation, which is confusing.
517  * We don't guarantee any specific ordering in general, though.
518  */
519  if (scan->rs_syncscan)
520  ss_report_location(scan->rs_rd, blockno);
521 
522  finished = (blockno == scan->rs_startblock);
523  }
524 
525  /*
526  * Reached end of scan?
527  */
528  if (finished)
529  {
530  if (BufferIsValid(scan->rs_cbuf))
531  ReleaseBuffer(scan->rs_cbuf);
532  scan->rs_cbuf = InvalidBuffer;
534  tuple->t_data = NULL;
535  scan->rs_inited = false;
536  return NULL;
537  }
538 
539  Assert(blockno < scan->rs_nblocks);
540  heapgetpage(scan, blockno);
541 
542  /* Re-establish state for new page */
543  if (!pagemode)
545 
546  page = (Page) BufferGetPage(scan->rs_cbuf);
547  all_visible = PageIsAllVisible(page) && !snapshot->takenDuringRecovery;
548  maxoffset = PageGetMaxOffsetNumber(page);
549  }
550 
551  /* Count successfully-fetched tuples as heap fetches */
553 
554  return &(scan->rs_ctup);
555 }
556 
557 /*
558  * Check visibility of the tuple.
559  */
560 static bool
562 {
563  if (scan->rs_pageatatime)
564  {
565  /*
566  * In pageatatime mode, heapgetpage() already did visibility checks,
567  * so just look at the info it left in rs_vistuples[].
568  *
569  * We use a binary search over the known-sorted array. Note: we could
570  * save some effort if we insisted that NextSampleTuple select tuples
571  * in increasing order, but it's not clear that there would be enough
572  * gain to justify the restriction.
573  */
574  int start = 0,
575  end = scan->rs_ntuples - 1;
576 
577  while (start <= end)
578  {
579  int mid = (start + end) / 2;
580  OffsetNumber curoffset = scan->rs_vistuples[mid];
581 
582  if (tupoffset == curoffset)
583  return true;
584  else if (tupoffset < curoffset)
585  end = mid - 1;
586  else
587  start = mid + 1;
588  }
589 
590  return false;
591  }
592  else
593  {
594  /* Otherwise, we have to check the tuple individually. */
595  return HeapTupleSatisfiesVisibility(tuple,
596  scan->rs_snapshot,
597  scan->rs_cbuf);
598  }
599 }
InitSampleScan_function InitSampleScan
Definition: tsmapi.h:70
#define DatumGetUInt32(X)
Definition: postgres.h:494
TupleTableSlot * ExecStoreTuple(HeapTuple tuple, TupleTableSlot *slot, Buffer buffer, bool shouldFree)
Definition: execTuples.c:320
#define BUFFER_LOCK_UNLOCK
Definition: bufmgr.h:87
static bool SampleTupleVisible(HeapTuple tuple, OffsetNumber tupoffset, HeapScanDesc scan)
List * qual
Definition: plannodes.h:130
BlockNumber rs_cblock
Definition: relscan.h:69
Plan plan
Definition: plannodes.h:305
TupleTableSlot * ExecSampleScan(SampleScanState *node)
void ExecInitScanTupleSlot(EState *estate, ScanState *scanstate)
Definition: execTuples.c:842
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Definition: heapam.c:1581
#define PageIsAllVisible(page)
Definition: bufpage.h:382
#define RelationGetDescr(relation)
Definition: rel.h:425
HeapTupleHeaderData * HeapTupleHeader
Definition: htup.h:23
long random(void)
Definition: random.c:22
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Definition: execScan.c:121
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Definition: execTuples.c:439
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Definition: execnodes.h:1062
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Definition: buf.h:25
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Definition: heapam.c:348
static TupleTableSlot * SampleNext(SampleScanState *node)
List * targetlist
Definition: execnodes.h:1061
Snapshot es_snapshot
Definition: execnodes.h:370
TupleTableSlot * ss_ScanTupleSlot
Definition: execnodes.h:1291
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Definition: block.h:31
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Definition: bufmgr.c:3292
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Definition: fmgr.h:555
Relation ss_currentRelation
Definition: execnodes.h:1289
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Definition: execnodes.h:1049
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Definition: bufpage.h:354
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Definition: execUtils.c:685
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Definition: predicate.c:3862
Datum ExecEvalExprSwitchContext(ExprState *expression, ExprContext *econtext, bool *isNull)
Definition: execQual.c:4219
void ExecAssignResultTypeFromTL(PlanState *planstate)
Definition: execUtils.c:430
#define HeapTupleSatisfiesVisibility(tuple, snapshot, buffer)
Definition: tqual.h:45
HeapTupleData rs_ctup
Definition: relscan.h:68
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Definition: off.h:24
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Definition: executor.h:271
HeapTupleHeader t_data
Definition: htup.h:67
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Definition: execnodes.h:1288
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Definition: executor.h:272
TupleTableSlot * ps_ResultTupleSlot
Definition: execnodes.h:1077
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Definition: execQual.c:4266
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Definition: itemid.h:58
void pfree(void *pointer)
Definition: mcxt.c:950
NextSampleTuple_function NextSampleTuple
Definition: tsmapi.h:73
#define ERROR
Definition: elog.h:43
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Definition: execScan.c:235
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Definition: heapam.c:1430
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Definition: relscan.h:53
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Scan scan
Definition: plannodes.h:321
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Definition: htup.h:65
#define innerPlan(node)
Definition: plannodes.h:158
uint32 t_len
Definition: htup.h:64
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Snapshot rs_snapshot
Definition: relscan.h:48
unsigned int uint32
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Definition: relscan.h:67
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Definition: tsmapi.h:72
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Definition: relscan.h:60
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Definition: plannodes.h:159
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Definition: hashfunc.c:120
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#define PageGetItemId(page, offsetNumber)
Definition: bufpage.h:232
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Definition: postgres.h:374
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Definition: relscan.h:59
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Definition: relscan.h:47
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Definition: block.h:70
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Definition: relscan.h:70
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OffsetNumber rs_vistuples[MaxHeapTuplesPerPage]
Definition: relscan.h:77
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Definition: itemid.h:98
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int i
#define OffsetNumberIsValid(offsetNumber)
Definition: off.h:40
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#define pgstat_count_heap_getnext(rel)
Definition: pgstat.h:1145
EndSampleScan_function EndSampleScan
Definition: tsmapi.h:74
Definition: pg_list.h:45
#define PageGetItem(page, itemId)
Definition: bufpage.h:337
Pointer Page
Definition: bufpage.h:74
#define ItemPointerSet(pointer, blockNumber, offNum)
Definition: itemptr.h:86
ScanState ss
Definition: execnodes.h:1310