Age Owner Branch data TLA Line data Source code
1 : : /*-------------------------------------------------------------------------
2 : : *
3 : : * tsm_system_time.c
4 : : * support routines for SYSTEM_TIME tablesample method
5 : : *
6 : : * The desire here is to produce a random sample with as many rows as possible
7 : : * in no more than the specified amount of time. 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 time dependence, this method is necessarily unrepeatable.
13 : : * However, we do what we can to reduce surprising behavior by selecting
14 : : * the sampling pattern just once per query, much as in tsm_system_rows.
15 : : *
16 : : * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
17 : : * Portions Copyright (c) 1994, Regents of the University of California
18 : : *
19 : : * IDENTIFICATION
20 : : * contrib/tsm_system_time/tsm_system_time.c
21 : : *
22 : : *-------------------------------------------------------------------------
23 : : */
24 : :
25 : : #include "postgres.h"
26 : :
27 : : #include <math.h>
28 : :
29 : : #include "access/relscan.h"
30 : : #include "access/tsmapi.h"
31 : : #include "catalog/pg_type.h"
32 : : #include "miscadmin.h"
33 : : #include "optimizer/optimizer.h"
34 : : #include "utils/sampling.h"
35 : : #include "utils/spccache.h"
36 : :
3257 simon@2ndQuadrant.co 37 :CBC 1 : PG_MODULE_MAGIC;
38 : :
3186 tgl@sss.pgh.pa.us 39 : 2 : PG_FUNCTION_INFO_V1(tsm_system_time_handler);
40 : :
41 : :
42 : : /* Private state */
43 : : typedef struct
44 : : {
45 : : uint32 seed; /* random seed */
46 : : double millis; /* time limit for sampling */
47 : : instr_time start_time; /* scan start time */
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 : : } SystemTimeSamplerData;
56 : :
57 : : static void system_time_samplescangetsamplesize(PlannerInfo *root,
58 : : RelOptInfo *baserel,
59 : : List *paramexprs,
60 : : BlockNumber *pages,
61 : : double *tuples);
62 : : static void system_time_initsamplescan(SampleScanState *node,
63 : : int eflags);
64 : : static void system_time_beginsamplescan(SampleScanState *node,
65 : : Datum *params,
66 : : int nparams,
67 : : uint32 seed);
68 : : static BlockNumber system_time_nextsampleblock(SampleScanState *node, BlockNumber nblocks);
69 : : static OffsetNumber system_time_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_TIME method.
77 : : */
78 : : Datum
79 : 41 : tsm_system_time_handler(PG_FUNCTION_ARGS)
80 : : {
81 : 41 : TsmRoutine *tsm = makeNode(TsmRoutine);
82 : :
83 : 41 : tsm->parameterTypes = list_make1_oid(FLOAT8OID);
84 : :
85 : : /* See notes at head of file */
86 : 41 : tsm->repeatable_across_queries = false;
87 : 41 : tsm->repeatable_across_scans = false;
88 : :
89 : 41 : tsm->SampleScanGetSampleSize = system_time_samplescangetsamplesize;
90 : 41 : tsm->InitSampleScan = system_time_initsamplescan;
91 : 41 : tsm->BeginSampleScan = system_time_beginsamplescan;
92 : 41 : tsm->NextSampleBlock = system_time_nextsampleblock;
93 : 41 : tsm->NextSampleTuple = system_time_nextsampletuple;
94 : 41 : tsm->EndSampleScan = NULL;
95 : :
96 : 41 : PG_RETURN_POINTER(tsm);
97 : : }
98 : :
99 : : /*
100 : : * Sample size estimation.
101 : : */
102 : : static void
103 : 9 : system_time_samplescangetsamplesize(PlannerInfo *root,
104 : : RelOptInfo *baserel,
105 : : List *paramexprs,
106 : : BlockNumber *pages,
107 : : double *tuples)
108 : : {
109 : : Node *limitnode;
110 : : double millis;
111 : : double spc_random_page_cost;
112 : : double npages;
113 : : double ntuples;
114 : :
115 : : /* Try to extract an estimate for the limit time spec */
116 : 9 : limitnode = (Node *) linitial(paramexprs);
117 : 9 : limitnode = estimate_expression_value(root, limitnode);
118 : :
119 [ + + ]: 9 : if (IsA(limitnode, Const) &&
120 [ + - ]: 7 : !((Const *) limitnode)->constisnull)
121 : : {
122 : 7 : millis = DatumGetFloat8(((Const *) limitnode)->constvalue);
123 [ + + - + ]: 7 : if (millis < 0 || isnan(millis))
124 : : {
125 : : /* Default millis if the value is bogus */
126 : 2 : millis = 1000;
127 : : }
128 : : }
129 : : else
130 : : {
131 : : /* Default millis if we didn't obtain a non-null Const */
132 : 2 : millis = 1000;
133 : : }
134 : :
135 : : /* Get the planner's idea of cost per page read */
136 : 9 : get_tablespace_page_costs(baserel->reltablespace,
137 : : &spc_random_page_cost,
138 : : NULL);
139 : :
140 : : /*
141 : : * Estimate the number of pages we can read by assuming that the cost
142 : : * figure is expressed in milliseconds. This is completely, unmistakably
143 : : * bogus, but we have to do something to produce an estimate and there's
144 : : * no better answer.
145 : : */
146 [ + - ]: 9 : if (spc_random_page_cost > 0)
147 : 9 : npages = millis / spc_random_page_cost;
148 : : else
3186 tgl@sss.pgh.pa.us 149 :UBC 0 : npages = millis; /* even more bogus, but whatcha gonna do? */
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 : if (baserel->tuples > 0 && baserel->pages > 0)
155 : 9 : {
156 : : /* Estimate number of tuples returned based on tuple density */
157 : 9 : double density = baserel->tuples / (double) baserel->pages;
158 : :
159 : 9 : ntuples = npages * density;
160 : : }
161 : : else
162 : : {
163 : : /* For lack of data, assume one tuple per page */
3186 tgl@sss.pgh.pa.us 164 :UBC 0 : ntuples = npages;
165 : : }
166 : :
167 : : /* Clamp to the estimated relation size */
3186 tgl@sss.pgh.pa.us 168 [ - + ]:CBC 9 : ntuples = clamp_row_est(Min(baserel->tuples, ntuples));
169 : :
170 : 9 : *pages = npages;
171 : 9 : *tuples = ntuples;
3257 simon@2ndQuadrant.co 172 : 9 : }
173 : :
174 : : /*
175 : : * Initialize during executor setup.
176 : : */
177 : : static void
3186 tgl@sss.pgh.pa.us 178 : 9 : system_time_initsamplescan(SampleScanState *node, int eflags)
179 : : {
180 : 9 : node->tsm_state = palloc0(sizeof(SystemTimeSamplerData));
181 : : /* Note the above leaves tsm_state->step equal to zero */
3257 simon@2ndQuadrant.co 182 : 9 : }
183 : :
184 : : /*
185 : : * Examine parameters and prepare for a sample scan.
186 : : */
187 : : static void
3186 tgl@sss.pgh.pa.us 188 : 6 : system_time_beginsamplescan(SampleScanState *node,
189 : : Datum *params,
190 : : int nparams,
191 : : uint32 seed)
192 : : {
193 : 6 : SystemTimeSamplerData *sampler = (SystemTimeSamplerData *) node->tsm_state;
194 : 6 : double millis = DatumGetFloat8(params[0]);
195 : :
196 [ + + - + ]: 6 : if (millis < 0 || isnan(millis))
197 [ + - ]: 1 : ereport(ERROR,
198 : : (errcode(ERRCODE_INVALID_TABLESAMPLE_ARGUMENT),
199 : : errmsg("sample collection time must not be negative")));
200 : :
201 : 5 : sampler->seed = seed;
202 : 5 : sampler->millis = millis;
3257 simon@2ndQuadrant.co 203 : 5 : sampler->lt = InvalidOffsetNumber;
204 : 5 : sampler->doneblocks = 0;
205 : : /* start_time, lb will be initialized during first NextSampleBlock call */
206 : : /* we intentionally do not change nblocks/firstblock/step here */
207 : 5 : }
208 : :
209 : : /*
210 : : * Select next block to sample.
211 : : *
212 : : * Uses linear probing algorithm for picking next block.
213 : : */
214 : : static BlockNumber
1842 andres@anarazel.de 215 : 26 : system_time_nextsampleblock(SampleScanState *node, BlockNumber nblocks)
216 : : {
3186 tgl@sss.pgh.pa.us 217 : 26 : SystemTimeSamplerData *sampler = (SystemTimeSamplerData *) node->tsm_state;
218 : : instr_time cur_time;
219 : :
220 : : /* First call within scan? */
221 [ + + ]: 26 : if (sampler->doneblocks == 0)
222 : : {
223 : : /* First scan within query? */
224 [ + + ]: 5 : if (sampler->step == 0)
225 : : {
226 : : /* Initialize now that we have scan descriptor */
227 : : pg_prng_state randstate;
228 : :
229 : : /* If relation is empty, there's nothing to scan */
1842 andres@anarazel.de 230 [ - + ]: 4 : if (nblocks == 0)
3186 tgl@sss.pgh.pa.us 231 :UBC 0 : return InvalidBlockNumber;
232 : :
233 : : /* We only need an RNG during this setup step */
868 tgl@sss.pgh.pa.us 234 :CBC 4 : sampler_random_init_state(sampler->seed, &randstate);
235 : :
236 : : /* Compute nblocks/firstblock/step only once per query */
1842 andres@anarazel.de 237 : 4 : sampler->nblocks = nblocks;
238 : :
239 : : /* Choose random starting block within the relation */
240 : : /* (Actually this is the predecessor of the first block visited) */
868 tgl@sss.pgh.pa.us 241 : 4 : sampler->firstblock = sampler_random_fract(&randstate) *
3186 242 : 4 : sampler->nblocks;
243 : :
244 : : /* Find relative prime as step size for linear probing */
868 245 : 4 : sampler->step = random_relative_prime(sampler->nblocks, &randstate);
246 : : }
247 : :
248 : : /* Reinitialize lb and start_time */
3186 249 : 5 : sampler->lb = sampler->firstblock;
250 : 5 : INSTR_TIME_SET_CURRENT(sampler->start_time);
251 : : }
252 : :
253 : : /* If we've read all blocks in relation, we're done */
254 [ + + ]: 26 : if (++sampler->doneblocks > sampler->nblocks)
255 : 3 : return InvalidBlockNumber;
256 : :
257 : : /* If we've used up all the allotted time, we're done */
258 : 23 : INSTR_TIME_SET_CURRENT(cur_time);
259 : 23 : INSTR_TIME_SUBTRACT(cur_time, sampler->start_time);
260 [ + + ]: 23 : if (INSTR_TIME_GET_MILLISEC(cur_time) >= sampler->millis)
261 : 2 : return InvalidBlockNumber;
262 : :
263 : : /*
264 : : * It's probably impossible for scan->rs_nblocks to decrease between scans
265 : : * within a query; but just in case, loop until we select a block number
266 : : * less than scan->rs_nblocks. We don't care if scan->rs_nblocks has
267 : : * increased since the first scan.
268 : : */
269 : : do
270 : : {
271 : : /* Advance lb, using uint64 arithmetic to forestall overflow */
272 : 21 : sampler->lb = ((uint64) sampler->lb + sampler->step) % sampler->nblocks;
1842 andres@anarazel.de 273 [ - + ]: 21 : } while (sampler->lb >= nblocks);
274 : :
3186 tgl@sss.pgh.pa.us 275 : 21 : return sampler->lb;
276 : : }
277 : :
278 : : /*
279 : : * Select next sampled tuple in current block.
280 : : *
281 : : * In block sampling, we just want to sample all the tuples in each selected
282 : : * block.
283 : : *
284 : : * When we reach end of the block, return InvalidOffsetNumber which tells
285 : : * SampleScan to go to next block.
286 : : */
287 : : static OffsetNumber
288 : 114 : system_time_nextsampletuple(SampleScanState *node,
289 : : BlockNumber blockno,
290 : : OffsetNumber maxoffset)
291 : : {
292 : 114 : SystemTimeSamplerData *sampler = (SystemTimeSamplerData *) node->tsm_state;
293 : 114 : OffsetNumber tupoffset = sampler->lt;
294 : :
295 : : /* Advance to next possible offset on page */
296 [ + + ]: 114 : if (tupoffset == InvalidOffsetNumber)
297 : 21 : tupoffset = FirstOffsetNumber;
298 : : else
299 : 93 : tupoffset++;
300 : :
301 : : /* Done? */
302 [ + + ]: 114 : if (tupoffset > maxoffset)
303 : 21 : tupoffset = InvalidOffsetNumber;
304 : :
305 : 114 : sampler->lt = tupoffset;
306 : :
307 : 114 : return tupoffset;
308 : : }
309 : :
310 : : /*
311 : : * Compute greatest common divisor of two uint32's.
312 : : */
313 : : static uint32
3249 bruce@momjian.us 314 : 4 : gcd(uint32 a, uint32 b)
315 : : {
316 : : uint32 c;
317 : :
3257 simon@2ndQuadrant.co 318 [ + + ]: 15 : while (a != 0)
319 : : {
320 : 11 : c = a;
321 : 11 : a = b % a;
322 : 11 : b = c;
323 : : }
324 : :
325 : 4 : return b;
326 : : }
327 : :
328 : : /*
329 : : * Pick a random value less than and relatively prime to n, if possible
330 : : * (else return 1).
331 : : */
332 : : static uint32
868 tgl@sss.pgh.pa.us 333 : 4 : random_relative_prime(uint32 n, pg_prng_state *randstate)
334 : : {
335 : : uint32 r;
336 : :
337 : : /* Safety check to avoid infinite loop or zero result for small n. */
3186 338 [ - + ]: 4 : if (n <= 1)
3186 tgl@sss.pgh.pa.us 339 :UBC 0 : return 1;
340 : :
341 : : /*
342 : : * This should only take 2 or 3 iterations as the probability of 2 numbers
343 : : * being relatively prime is ~61%; but just in case, we'll include a
344 : : * CHECK_FOR_INTERRUPTS in the loop.
345 : : */
346 : : do
347 : : {
3257 simon@2ndQuadrant.co 348 [ - + ]:CBC 6 : CHECK_FOR_INTERRUPTS();
3186 tgl@sss.pgh.pa.us 349 : 6 : r = (uint32) (sampler_random_fract(randstate) * n);
350 [ + + - + ]: 6 : } while (r == 0 || gcd(r, n) > 1);
351 : :
3257 simon@2ndQuadrant.co 352 : 4 : return r;
353 : : }
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