Line data Source code
1 : /*-------------------------------------------------------------------------
2 : *
3 : * nodeSamplescan.c
4 : * Support routines for sample scans of relations (table sampling).
5 : *
6 : * Portions Copyright (c) 1996-2023, 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/relscan.h"
18 : #include "access/tableam.h"
19 : #include "access/tsmapi.h"
20 : #include "common/pg_prng.h"
21 : #include "executor/executor.h"
22 : #include "executor/nodeSamplescan.h"
23 : #include "miscadmin.h"
24 : #include "pgstat.h"
25 : #include "storage/bufmgr.h"
26 : #include "storage/predicate.h"
27 : #include "utils/builtins.h"
28 : #include "utils/rel.h"
29 :
30 : static TupleTableSlot *SampleNext(SampleScanState *node);
31 : static void tablesample_init(SampleScanState *scanstate);
32 : static TupleTableSlot *tablesample_getnext(SampleScanState *scanstate);
33 :
34 : /* ----------------------------------------------------------------
35 : * Scan Support
36 : * ----------------------------------------------------------------
37 : */
38 :
39 : /* ----------------------------------------------------------------
40 : * SampleNext
41 : *
42 : * This is a workhorse for ExecSampleScan
43 : * ----------------------------------------------------------------
44 : */
45 : static TupleTableSlot *
46 241370 : SampleNext(SampleScanState *node)
47 : {
48 : /*
49 : * if this is first call within a scan, initialize
50 : */
51 241370 : if (!node->begun)
52 216 : tablesample_init(node);
53 :
54 : /*
55 : * get the next tuple, and store it in our result slot
56 : */
57 241330 : return tablesample_getnext(node);
58 : }
59 :
60 : /*
61 : * SampleRecheck -- access method routine to recheck a tuple in EvalPlanQual
62 : */
63 : static bool
64 0 : SampleRecheck(SampleScanState *node, TupleTableSlot *slot)
65 : {
66 : /*
67 : * No need to recheck for SampleScan, since like SeqScan we don't pass any
68 : * checkable keys to heap_beginscan.
69 : */
70 0 : return true;
71 : }
72 :
73 : /* ----------------------------------------------------------------
74 : * ExecSampleScan(node)
75 : *
76 : * Scans the relation using the sampling method and returns
77 : * the next qualifying tuple.
78 : * We call the ExecScan() routine and pass it the appropriate
79 : * access method functions.
80 : * ----------------------------------------------------------------
81 : */
82 : static TupleTableSlot *
83 241364 : ExecSampleScan(PlanState *pstate)
84 : {
85 241364 : SampleScanState *node = castNode(SampleScanState, pstate);
86 :
87 241364 : return ExecScan(&node->ss,
88 : (ExecScanAccessMtd) SampleNext,
89 : (ExecScanRecheckMtd) SampleRecheck);
90 : }
91 :
92 : /* ----------------------------------------------------------------
93 : * ExecInitSampleScan
94 : * ----------------------------------------------------------------
95 : */
96 : SampleScanState *
97 252 : ExecInitSampleScan(SampleScan *node, EState *estate, int eflags)
98 : {
99 : SampleScanState *scanstate;
100 252 : TableSampleClause *tsc = node->tablesample;
101 : TsmRoutine *tsm;
102 :
103 : Assert(outerPlan(node) == NULL);
104 : Assert(innerPlan(node) == NULL);
105 :
106 : /*
107 : * create state structure
108 : */
109 252 : scanstate = makeNode(SampleScanState);
110 252 : scanstate->ss.ps.plan = (Plan *) node;
111 252 : scanstate->ss.ps.state = estate;
112 252 : scanstate->ss.ps.ExecProcNode = ExecSampleScan;
113 :
114 : /*
115 : * Miscellaneous initialization
116 : *
117 : * create expression context for node
118 : */
119 252 : ExecAssignExprContext(estate, &scanstate->ss.ps);
120 :
121 : /*
122 : * open the scan relation
123 : */
124 252 : scanstate->ss.ss_currentRelation =
125 252 : ExecOpenScanRelation(estate,
126 : node->scan.scanrelid,
127 : eflags);
128 :
129 : /* we won't set up the HeapScanDesc till later */
130 252 : scanstate->ss.ss_currentScanDesc = NULL;
131 :
132 : /* and create slot with appropriate rowtype */
133 252 : ExecInitScanTupleSlot(estate, &scanstate->ss,
134 252 : RelationGetDescr(scanstate->ss.ss_currentRelation),
135 : table_slot_callbacks(scanstate->ss.ss_currentRelation));
136 :
137 : /*
138 : * Initialize result type and projection.
139 : */
140 252 : ExecInitResultTypeTL(&scanstate->ss.ps);
141 252 : ExecAssignScanProjectionInfo(&scanstate->ss);
142 :
143 : /*
144 : * initialize child expressions
145 : */
146 252 : scanstate->ss.ps.qual =
147 252 : ExecInitQual(node->scan.plan.qual, (PlanState *) scanstate);
148 :
149 252 : scanstate->args = ExecInitExprList(tsc->args, (PlanState *) scanstate);
150 252 : scanstate->repeatable =
151 252 : ExecInitExpr(tsc->repeatable, (PlanState *) scanstate);
152 :
153 : /*
154 : * If we don't have a REPEATABLE clause, select a random seed. We want to
155 : * do this just once, since the seed shouldn't change over rescans.
156 : */
157 252 : if (tsc->repeatable == NULL)
158 168 : scanstate->seed = pg_prng_uint32(&pg_global_prng_state);
159 :
160 : /*
161 : * Finally, initialize the TABLESAMPLE method handler.
162 : */
163 252 : tsm = GetTsmRoutine(tsc->tsmhandler);
164 252 : scanstate->tsmroutine = tsm;
165 252 : scanstate->tsm_state = NULL;
166 :
167 252 : if (tsm->InitSampleScan)
168 252 : tsm->InitSampleScan(scanstate, eflags);
169 :
170 : /* We'll do BeginSampleScan later; we can't evaluate params yet */
171 252 : scanstate->begun = false;
172 :
173 252 : return scanstate;
174 : }
175 :
176 : /* ----------------------------------------------------------------
177 : * ExecEndSampleScan
178 : *
179 : * frees any storage allocated through C routines.
180 : * ----------------------------------------------------------------
181 : */
182 : void
183 212 : ExecEndSampleScan(SampleScanState *node)
184 : {
185 : /*
186 : * Tell sampling function that we finished the scan.
187 : */
188 212 : if (node->tsmroutine->EndSampleScan)
189 0 : node->tsmroutine->EndSampleScan(node);
190 :
191 : /*
192 : * close heap scan
193 : */
194 212 : if (node->ss.ss_currentScanDesc)
195 146 : table_endscan(node->ss.ss_currentScanDesc);
196 212 : }
197 :
198 : /* ----------------------------------------------------------------
199 : * ExecReScanSampleScan
200 : *
201 : * Rescans the relation.
202 : *
203 : * ----------------------------------------------------------------
204 : */
205 : void
206 58 : ExecReScanSampleScan(SampleScanState *node)
207 : {
208 : /* Remember we need to do BeginSampleScan again (if we did it at all) */
209 58 : node->begun = false;
210 58 : node->done = false;
211 58 : node->haveblock = false;
212 58 : node->donetuples = 0;
213 :
214 58 : ExecScanReScan(&node->ss);
215 58 : }
216 :
217 :
218 : /*
219 : * Initialize the TABLESAMPLE method: evaluate params and call BeginSampleScan.
220 : */
221 : static void
222 216 : tablesample_init(SampleScanState *scanstate)
223 : {
224 216 : TsmRoutine *tsm = scanstate->tsmroutine;
225 216 : ExprContext *econtext = scanstate->ss.ps.ps_ExprContext;
226 : Datum *params;
227 : Datum datum;
228 : bool isnull;
229 : uint32 seed;
230 : bool allow_sync;
231 : int i;
232 : ListCell *arg;
233 :
234 216 : scanstate->donetuples = 0;
235 216 : params = (Datum *) palloc(list_length(scanstate->args) * sizeof(Datum));
236 :
237 216 : i = 0;
238 426 : foreach(arg, scanstate->args)
239 : {
240 216 : ExprState *argstate = (ExprState *) lfirst(arg);
241 :
242 216 : params[i] = ExecEvalExprSwitchContext(argstate,
243 : econtext,
244 : &isnull);
245 216 : if (isnull)
246 6 : ereport(ERROR,
247 : (errcode(ERRCODE_INVALID_TABLESAMPLE_ARGUMENT),
248 : errmsg("TABLESAMPLE parameter cannot be null")));
249 210 : i++;
250 : }
251 :
252 210 : if (scanstate->repeatable)
253 : {
254 72 : datum = ExecEvalExprSwitchContext(scanstate->repeatable,
255 : econtext,
256 : &isnull);
257 72 : if (isnull)
258 6 : ereport(ERROR,
259 : (errcode(ERRCODE_INVALID_TABLESAMPLE_REPEAT),
260 : errmsg("TABLESAMPLE REPEATABLE parameter cannot be null")));
261 :
262 : /*
263 : * The REPEATABLE parameter has been coerced to float8 by the parser.
264 : * The reason for using float8 at the SQL level is that it will
265 : * produce unsurprising results both for users used to databases that
266 : * accept only integers in the REPEATABLE clause and for those who
267 : * might expect that REPEATABLE works like setseed() (a float in the
268 : * range from -1 to 1).
269 : *
270 : * We use hashfloat8() to convert the supplied value into a suitable
271 : * seed. For regression-testing purposes, that has the convenient
272 : * property that REPEATABLE(0) gives a machine-independent result.
273 : */
274 66 : seed = DatumGetUInt32(DirectFunctionCall1(hashfloat8, datum));
275 : }
276 : else
277 : {
278 : /* Use the seed selected by ExecInitSampleScan */
279 138 : seed = scanstate->seed;
280 : }
281 :
282 : /* Set default values for params that BeginSampleScan can adjust */
283 204 : scanstate->use_bulkread = true;
284 204 : scanstate->use_pagemode = true;
285 :
286 : /* Let tablesample method do its thing */
287 204 : tsm->BeginSampleScan(scanstate,
288 : params,
289 204 : list_length(scanstate->args),
290 : seed);
291 :
292 : /* We'll use syncscan if there's no NextSampleBlock function */
293 176 : allow_sync = (tsm->NextSampleBlock == NULL);
294 :
295 : /* Now we can create or reset the HeapScanDesc */
296 176 : if (scanstate->ss.ss_currentScanDesc == NULL)
297 : {
298 146 : scanstate->ss.ss_currentScanDesc =
299 146 : table_beginscan_sampling(scanstate->ss.ss_currentRelation,
300 146 : scanstate->ss.ps.state->es_snapshot,
301 : 0, NULL,
302 146 : scanstate->use_bulkread,
303 : allow_sync,
304 146 : scanstate->use_pagemode);
305 : }
306 : else
307 : {
308 30 : table_rescan_set_params(scanstate->ss.ss_currentScanDesc, NULL,
309 30 : scanstate->use_bulkread,
310 : allow_sync,
311 30 : scanstate->use_pagemode);
312 : }
313 :
314 176 : pfree(params);
315 :
316 : /* And we're initialized. */
317 176 : scanstate->begun = true;
318 176 : }
319 :
320 : /*
321 : * Get next tuple from TABLESAMPLE method.
322 : */
323 : static TupleTableSlot *
324 241330 : tablesample_getnext(SampleScanState *scanstate)
325 : {
326 241330 : TableScanDesc scan = scanstate->ss.ss_currentScanDesc;
327 241330 : TupleTableSlot *slot = scanstate->ss.ss_ScanTupleSlot;
328 :
329 241330 : ExecClearTuple(slot);
330 :
331 241330 : if (scanstate->done)
332 0 : return NULL;
333 :
334 : for (;;)
335 : {
336 254060 : if (!scanstate->haveblock)
337 : {
338 12906 : if (!table_scan_sample_next_block(scan, scanstate))
339 : {
340 170 : scanstate->haveblock = false;
341 170 : scanstate->done = true;
342 :
343 : /* exhausted relation */
344 170 : return NULL;
345 : }
346 :
347 12736 : scanstate->haveblock = true;
348 : }
349 :
350 253890 : if (!table_scan_sample_next_tuple(scan, scanstate, slot))
351 : {
352 : /*
353 : * If we get here, it means we've exhausted the items on this page
354 : * and it's time to move to the next.
355 : */
356 12730 : scanstate->haveblock = false;
357 12730 : continue;
358 : }
359 :
360 : /* Found visible tuple, return it. */
361 241160 : break;
362 : }
363 :
364 241160 : scanstate->donetuples++;
365 :
366 241160 : return slot;
367 : }
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