Age Owner Branch data TLA Line data Source code
1 : : /*-------------------------------------------------------------------------
2 : : *
3 : : * tsm_system_rows.c
4 : : * support routines for SYSTEM_ROWS tablesample method
5 : : *
6 : : * The desire here is to produce a random sample with a given number of rows
7 : : * (or the whole relation, if that is fewer rows). We use a block-sampling
8 : : * approach. To ensure that the whole relation will be visited if necessary,
9 : : * we start at a randomly chosen block and then advance with a stride that
10 : : * is randomly chosen but is relatively prime to the relation's nblocks.
11 : : *
12 : : * Because of the dependence on nblocks, this method cannot be repeatable
13 : : * across queries. (Even if the user hasn't explicitly changed the relation,
14 : : * maintenance activities such as autovacuum might change nblocks.) However,
15 : : * we can at least make it repeatable across scans, by determining the
16 : : * sampling pattern only once on the first scan. This means that rescans
17 : : * won't visit blocks added after the first scan, but that is fine since
18 : : * such blocks shouldn't contain any visible tuples anyway.
19 : : *
20 : : * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
21 : : * Portions Copyright (c) 1994, Regents of the University of California
22 : : *
23 : : * IDENTIFICATION
24 : : * contrib/tsm_system_rows/tsm_system_rows.c
25 : : *
26 : : *-------------------------------------------------------------------------
27 : : */
28 : :
29 : : #include "postgres.h"
30 : :
31 : : #include "access/relscan.h"
32 : : #include "access/tsmapi.h"
33 : : #include "catalog/pg_type.h"
34 : : #include "miscadmin.h"
35 : : #include "optimizer/optimizer.h"
36 : : #include "utils/sampling.h"
37 : :
3257 simon@2ndQuadrant.co 38 :CBC 1 : PG_MODULE_MAGIC;
39 : :
3186 tgl@sss.pgh.pa.us 40 : 2 : PG_FUNCTION_INFO_V1(tsm_system_rows_handler);
41 : :
42 : :
43 : : /* Private state */
44 : : typedef struct
45 : : {
46 : : uint32 seed; /* random seed */
47 : : int64 ntuples; /* number of tuples to return */
48 : : OffsetNumber lt; /* last tuple returned from current block */
49 : : BlockNumber doneblocks; /* number of already-scanned blocks */
50 : : BlockNumber lb; /* last block visited */
51 : : /* these three values are not changed during a rescan: */
52 : : BlockNumber nblocks; /* number of blocks in relation */
53 : : BlockNumber firstblock; /* first block to sample from */
54 : : BlockNumber step; /* step size, or 0 if not set yet */
55 : : } SystemRowsSamplerData;
56 : :
57 : : static void system_rows_samplescangetsamplesize(PlannerInfo *root,
58 : : RelOptInfo *baserel,
59 : : List *paramexprs,
60 : : BlockNumber *pages,
61 : : double *tuples);
62 : : static void system_rows_initsamplescan(SampleScanState *node,
63 : : int eflags);
64 : : static void system_rows_beginsamplescan(SampleScanState *node,
65 : : Datum *params,
66 : : int nparams,
67 : : uint32 seed);
68 : : static BlockNumber system_rows_nextsampleblock(SampleScanState *node, BlockNumber nblocks);
69 : : static OffsetNumber system_rows_nextsampletuple(SampleScanState *node,
70 : : BlockNumber blockno,
71 : : OffsetNumber maxoffset);
72 : : static uint32 random_relative_prime(uint32 n, pg_prng_state *randstate);
73 : :
74 : :
75 : : /*
76 : : * Create a TsmRoutine descriptor for the SYSTEM_ROWS method.
77 : : */
78 : : Datum
79 : 40 : tsm_system_rows_handler(PG_FUNCTION_ARGS)
80 : : {
81 : 40 : TsmRoutine *tsm = makeNode(TsmRoutine);
82 : :
83 : 40 : tsm->parameterTypes = list_make1_oid(INT8OID);
84 : :
85 : : /* See notes at head of file */
86 : 40 : tsm->repeatable_across_queries = false;
87 : 40 : tsm->repeatable_across_scans = true;
88 : :
89 : 40 : tsm->SampleScanGetSampleSize = system_rows_samplescangetsamplesize;
90 : 40 : tsm->InitSampleScan = system_rows_initsamplescan;
91 : 40 : tsm->BeginSampleScan = system_rows_beginsamplescan;
92 : 40 : tsm->NextSampleBlock = system_rows_nextsampleblock;
93 : 40 : tsm->NextSampleTuple = system_rows_nextsampletuple;
94 : 40 : tsm->EndSampleScan = NULL;
95 : :
96 : 40 : PG_RETURN_POINTER(tsm);
97 : : }
98 : :
99 : : /*
100 : : * Sample size estimation.
101 : : */
102 : : static void
103 : 9 : system_rows_samplescangetsamplesize(PlannerInfo *root,
104 : : RelOptInfo *baserel,
105 : : List *paramexprs,
106 : : BlockNumber *pages,
107 : : double *tuples)
108 : : {
109 : : Node *limitnode;
110 : : int64 ntuples;
111 : : double npages;
112 : :
113 : : /* Try to extract an estimate for the limit rowcount */
114 : 9 : limitnode = (Node *) linitial(paramexprs);
115 : 9 : limitnode = estimate_expression_value(root, limitnode);
116 : :
117 [ + + ]: 9 : if (IsA(limitnode, Const) &&
118 [ + - ]: 7 : !((Const *) limitnode)->constisnull)
119 : : {
120 : 7 : ntuples = DatumGetInt64(((Const *) limitnode)->constvalue);
121 [ + + ]: 7 : if (ntuples < 0)
122 : : {
123 : : /* Default ntuples if the value is bogus */
124 : 2 : ntuples = 1000;
125 : : }
126 : : }
127 : : else
128 : : {
129 : : /* Default ntuples if we didn't obtain a non-null Const */
130 : 2 : ntuples = 1000;
131 : : }
132 : :
133 : : /* Clamp to the estimated relation size */
134 [ + + ]: 9 : if (ntuples > baserel->tuples)
135 : 5 : ntuples = (int64) baserel->tuples;
136 : 9 : ntuples = clamp_row_est(ntuples);
137 : :
138 [ + - + - ]: 9 : if (baserel->tuples > 0 && baserel->pages > 0)
139 : 9 : {
140 : : /* Estimate number of pages visited based on tuple density */
141 : 9 : double density = baserel->tuples / (double) baserel->pages;
142 : :
143 : 9 : npages = ntuples / density;
144 : : }
145 : : else
146 : : {
147 : : /* For lack of data, assume one tuple per page */
3186 tgl@sss.pgh.pa.us 148 :UBC 0 : npages = ntuples;
149 : : }
150 : :
151 : : /* Clamp to sane value */
3186 tgl@sss.pgh.pa.us 152 [ - + ]:CBC 9 : npages = clamp_row_est(Min((double) baserel->pages, npages));
153 : :
154 : 9 : *pages = npages;
155 : 9 : *tuples = ntuples;
3257 simon@2ndQuadrant.co 156 : 9 : }
157 : :
158 : : /*
159 : : * Initialize during executor setup.
160 : : */
161 : : static void
3186 tgl@sss.pgh.pa.us 162 : 9 : system_rows_initsamplescan(SampleScanState *node, int eflags)
163 : : {
164 : 9 : node->tsm_state = palloc0(sizeof(SystemRowsSamplerData));
165 : : /* Note the above leaves tsm_state->step equal to zero */
3257 simon@2ndQuadrant.co 166 : 9 : }
167 : :
168 : : /*
169 : : * Examine parameters and prepare for a sample scan.
170 : : */
171 : : static void
3186 tgl@sss.pgh.pa.us 172 : 9 : system_rows_beginsamplescan(SampleScanState *node,
173 : : Datum *params,
174 : : int nparams,
175 : : uint32 seed)
176 : : {
177 : 9 : SystemRowsSamplerData *sampler = (SystemRowsSamplerData *) node->tsm_state;
178 : 9 : int64 ntuples = DatumGetInt64(params[0]);
179 : :
180 [ + + ]: 9 : if (ntuples < 0)
181 [ + - ]: 1 : ereport(ERROR,
182 : : (errcode(ERRCODE_INVALID_TABLESAMPLE_ARGUMENT),
183 : : errmsg("sample size must not be negative")));
184 : :
185 : 8 : sampler->seed = seed;
186 : 8 : sampler->ntuples = ntuples;
187 : 8 : sampler->lt = InvalidOffsetNumber;
188 : 8 : sampler->doneblocks = 0;
189 : : /* lb will be initialized during first NextSampleBlock call */
190 : : /* we intentionally do not change nblocks/firstblock/step here */
191 : :
192 : : /*
193 : : * We *must* use pagemode visibility checking in this module, so force
194 : : * that even though it's currently default.
195 : : */
196 : 8 : node->use_pagemode = true;
3257 simon@2ndQuadrant.co 197 : 8 : }
198 : :
199 : : /*
200 : : * Select next block to sample.
201 : : *
202 : : * Uses linear probing algorithm for picking next block.
203 : : */
204 : : static BlockNumber
1842 andres@anarazel.de 205 : 32 : system_rows_nextsampleblock(SampleScanState *node, BlockNumber nblocks)
206 : : {
3186 tgl@sss.pgh.pa.us 207 : 32 : SystemRowsSamplerData *sampler = (SystemRowsSamplerData *) node->tsm_state;
208 : :
209 : : /* First call within scan? */
210 [ + + ]: 32 : if (sampler->doneblocks == 0)
211 : : {
212 : : /* First scan within query? */
213 [ + + ]: 8 : if (sampler->step == 0)
214 : : {
215 : : /* Initialize now that we have scan descriptor */
216 : : pg_prng_state randstate;
217 : :
218 : : /* If relation is empty, there's nothing to scan */
1842 andres@anarazel.de 219 [ - + ]: 6 : if (nblocks == 0)
3186 tgl@sss.pgh.pa.us 220 :UBC 0 : return InvalidBlockNumber;
221 : :
222 : : /* We only need an RNG during this setup step */
868 tgl@sss.pgh.pa.us 223 :CBC 6 : sampler_random_init_state(sampler->seed, &randstate);
224 : :
225 : : /* Compute nblocks/firstblock/step only once per query */
1842 andres@anarazel.de 226 : 6 : sampler->nblocks = nblocks;
227 : :
228 : : /* Choose random starting block within the relation */
229 : : /* (Actually this is the predecessor of the first block visited) */
868 tgl@sss.pgh.pa.us 230 : 6 : sampler->firstblock = sampler_random_fract(&randstate) *
3186 231 : 6 : sampler->nblocks;
232 : :
233 : : /* Find relative prime as step size for linear probing */
868 234 : 6 : sampler->step = random_relative_prime(sampler->nblocks, &randstate);
235 : : }
236 : :
237 : : /* Reinitialize lb */
3186 238 : 8 : sampler->lb = sampler->firstblock;
239 : : }
240 : :
241 : : /* If we've read all blocks or returned all needed tuples, we're done */
242 [ + + ]: 32 : if (++sampler->doneblocks > sampler->nblocks ||
1842 andres@anarazel.de 243 [ + + ]: 30 : node->donetuples >= sampler->ntuples)
3186 tgl@sss.pgh.pa.us 244 : 8 : return InvalidBlockNumber;
245 : :
246 : : /*
247 : : * It's probably impossible for scan->rs_nblocks to decrease between scans
248 : : * within a query; but just in case, loop until we select a block number
249 : : * less than scan->rs_nblocks. We don't care if scan->rs_nblocks has
250 : : * increased since the first scan.
251 : : */
252 : : do
253 : : {
254 : : /* Advance lb, using uint64 arithmetic to forestall overflow */
255 : 24 : sampler->lb = ((uint64) sampler->lb + sampler->step) % sampler->nblocks;
1842 andres@anarazel.de 256 [ - + ]: 24 : } while (sampler->lb >= nblocks);
257 : :
3186 tgl@sss.pgh.pa.us 258 : 24 : return sampler->lb;
259 : : }
260 : :
261 : : /*
262 : : * Select next sampled tuple in current block.
263 : : *
264 : : * In block sampling, we just want to sample all the tuples in each selected
265 : : * block.
266 : : *
267 : : * When we reach end of the block, return InvalidOffsetNumber which tells
268 : : * SampleScan to go to next block.
269 : : */
270 : : static OffsetNumber
271 : 127 : system_rows_nextsampletuple(SampleScanState *node,
272 : : BlockNumber blockno,
273 : : OffsetNumber maxoffset)
274 : : {
275 : 127 : SystemRowsSamplerData *sampler = (SystemRowsSamplerData *) node->tsm_state;
276 : 127 : OffsetNumber tupoffset = sampler->lt;
277 : :
278 : : /* Quit if we've returned all needed tuples */
1842 andres@anarazel.de 279 [ + + ]: 127 : if (node->donetuples >= sampler->ntuples)
3186 tgl@sss.pgh.pa.us 280 : 4 : return InvalidOffsetNumber;
281 : :
282 : : /* Advance to next possible offset on page */
1842 andres@anarazel.de 283 [ + + ]: 123 : if (tupoffset == InvalidOffsetNumber)
284 : 24 : tupoffset = FirstOffsetNumber;
285 : : else
286 : 99 : tupoffset++;
287 : :
288 : : /* Done? */
289 [ + + ]: 123 : if (tupoffset > maxoffset)
290 : 20 : tupoffset = InvalidOffsetNumber;
291 : :
3186 tgl@sss.pgh.pa.us 292 : 123 : sampler->lt = tupoffset;
293 : :
294 : 123 : return tupoffset;
295 : : }
296 : :
297 : : /*
298 : : * Compute greatest common divisor of two uint32's.
299 : : */
300 : : static uint32
3249 bruce@momjian.us 301 : 6 : gcd(uint32 a, uint32 b)
302 : : {
303 : : uint32 c;
304 : :
3257 simon@2ndQuadrant.co 305 [ + + ]: 20 : while (a != 0)
306 : : {
307 : 14 : c = a;
308 : 14 : a = b % a;
309 : 14 : b = c;
310 : : }
311 : :
312 : 6 : return b;
313 : : }
314 : :
315 : : /*
316 : : * Pick a random value less than and relatively prime to n, if possible
317 : : * (else return 1).
318 : : */
319 : : static uint32
868 tgl@sss.pgh.pa.us 320 : 6 : random_relative_prime(uint32 n, pg_prng_state *randstate)
321 : : {
322 : : uint32 r;
323 : :
324 : : /* Safety check to avoid infinite loop or zero result for small n. */
3186 325 [ - + ]: 6 : if (n <= 1)
3186 tgl@sss.pgh.pa.us 326 :UBC 0 : return 1;
327 : :
328 : : /*
329 : : * This should only take 2 or 3 iterations as the probability of 2 numbers
330 : : * being relatively prime is ~61%; but just in case, we'll include a
331 : : * CHECK_FOR_INTERRUPTS in the loop.
332 : : */
333 : : do
334 : : {
3257 simon@2ndQuadrant.co 335 [ - + ]:CBC 9 : CHECK_FOR_INTERRUPTS();
3186 tgl@sss.pgh.pa.us 336 : 9 : r = (uint32) (sampler_random_fract(randstate) * n);
337 [ + + - + ]: 9 : } while (r == 0 || gcd(r, n) > 1);
338 : :
3257 simon@2ndQuadrant.co 339 : 6 : return r;
340 : : }
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