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tsm_system_rows.c
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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) 1996-2019, 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 
39 
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 */
56 
58  RelOptInfo *baserel,
59  List *paramexprs,
60  BlockNumber *pages,
61  double *tuples);
63  int eflags);
65  Datum *params,
66  int nparams,
67  uint32 seed);
70  BlockNumber blockno,
71  OffsetNumber maxoffset);
73 
74 
75 /*
76  * Create a TsmRoutine descriptor for the SYSTEM_ROWS method.
77  */
78 Datum
80 {
82 
83  tsm->parameterTypes = list_make1_oid(INT8OID);
84 
85  /* See notes at head of file */
86  tsm->repeatable_across_queries = false;
87  tsm->repeatable_across_scans = true;
88 
94  tsm->EndSampleScan = NULL;
95 
96  PG_RETURN_POINTER(tsm);
97 }
98 
99 /*
100  * Sample size estimation.
101  */
102 static void
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  limitnode = (Node *) linitial(paramexprs);
115  limitnode = estimate_expression_value(root, limitnode);
116 
117  if (IsA(limitnode, Const) &&
118  !((Const *) limitnode)->constisnull)
119  {
120  ntuples = DatumGetInt64(((Const *) limitnode)->constvalue);
121  if (ntuples < 0)
122  {
123  /* Default ntuples if the value is bogus */
124  ntuples = 1000;
125  }
126  }
127  else
128  {
129  /* Default ntuples if we didn't obtain a non-null Const */
130  ntuples = 1000;
131  }
132 
133  /* Clamp to the estimated relation size */
134  if (ntuples > baserel->tuples)
135  ntuples = (int64) baserel->tuples;
136  ntuples = clamp_row_est(ntuples);
137 
138  if (baserel->tuples > 0 && baserel->pages > 0)
139  {
140  /* Estimate number of pages visited based on tuple density */
141  double density = baserel->tuples / (double) baserel->pages;
142 
143  npages = ntuples / density;
144  }
145  else
146  {
147  /* For lack of data, assume one tuple per page */
148  npages = ntuples;
149  }
150 
151  /* Clamp to sane value */
152  npages = clamp_row_est(Min((double) baserel->pages, npages));
153 
154  *pages = npages;
155  *tuples = ntuples;
156 }
157 
158 /*
159  * Initialize during executor setup.
160  */
161 static void
163 {
164  node->tsm_state = palloc0(sizeof(SystemRowsSamplerData));
165  /* Note the above leaves tsm_state->step equal to zero */
166 }
167 
168 /*
169  * Examine parameters and prepare for a sample scan.
170  */
171 static void
173  Datum *params,
174  int nparams,
175  uint32 seed)
176 {
178  int64 ntuples = DatumGetInt64(params[0]);
179 
180  if (ntuples < 0)
181  ereport(ERROR,
182  (errcode(ERRCODE_INVALID_TABLESAMPLE_ARGUMENT),
183  errmsg("sample size must not be negative")));
184 
185  sampler->seed = seed;
186  sampler->ntuples = ntuples;
187  sampler->lt = InvalidOffsetNumber;
188  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  node->use_pagemode = true;
197 }
198 
199 /*
200  * Select next block to sample.
201  *
202  * Uses linear probing algorithm for picking next block.
203  */
204 static BlockNumber
206 {
208 
209  /* First call within scan? */
210  if (sampler->doneblocks == 0)
211  {
212  /* First scan within query? */
213  if (sampler->step == 0)
214  {
215  /* Initialize now that we have scan descriptor */
216  SamplerRandomState randstate;
217 
218  /* If relation is empty, there's nothing to scan */
219  if (nblocks == 0)
220  return InvalidBlockNumber;
221 
222  /* We only need an RNG during this setup step */
223  sampler_random_init_state(sampler->seed, randstate);
224 
225  /* Compute nblocks/firstblock/step only once per query */
226  sampler->nblocks = nblocks;
227 
228  /* Choose random starting block within the relation */
229  /* (Actually this is the predecessor of the first block visited) */
230  sampler->firstblock = sampler_random_fract(randstate) *
231  sampler->nblocks;
232 
233  /* Find relative prime as step size for linear probing */
234  sampler->step = random_relative_prime(sampler->nblocks, randstate);
235  }
236 
237  /* Reinitialize lb */
238  sampler->lb = sampler->firstblock;
239  }
240 
241  /* If we've read all blocks or returned all needed tuples, we're done */
242  if (++sampler->doneblocks > sampler->nblocks ||
243  node->donetuples >= sampler->ntuples)
244  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  sampler->lb = ((uint64) sampler->lb + sampler->step) % sampler->nblocks;
256  } while (sampler->lb >= nblocks);
257 
258  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
272  BlockNumber blockno,
273  OffsetNumber maxoffset)
274 {
276  OffsetNumber tupoffset = sampler->lt;
277 
278  /* Quit if we've returned all needed tuples */
279  if (node->donetuples >= sampler->ntuples)
280  return InvalidOffsetNumber;
281 
282  /* Advance to next possible offset on page */
283  if (tupoffset == InvalidOffsetNumber)
284  tupoffset = FirstOffsetNumber;
285  else
286  tupoffset++;
287 
288  /* Done? */
289  if (tupoffset > maxoffset)
290  tupoffset = InvalidOffsetNumber;
291 
292  sampler->lt = tupoffset;
293 
294  return tupoffset;
295 }
296 
297 /*
298  * Compute greatest common divisor of two uint32's.
299  */
300 static uint32
302 {
303  uint32 c;
304 
305  while (a != 0)
306  {
307  c = a;
308  a = b % a;
309  b = c;
310  }
311 
312  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
321 {
322  uint32 r;
323 
324  /* Safety check to avoid infinite loop or zero result for small n. */
325  if (n <= 1)
326  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  {
336  r = (uint32) (sampler_random_fract(randstate) * n);
337  } while (r == 0 || gcd(r, n) > 1);
338 
339  return r;
340 }
Datum tsm_system_rows_handler(PG_FUNCTION_ARGS)
InitSampleScan_function InitSampleScan
Definition: tsmapi.h:71
#define PG_RETURN_POINTER(x)
Definition: fmgr.h:351
#define IsA(nodeptr, _type_)
Definition: nodes.h:576
bool repeatable_across_queries
Definition: tsmapi.h:64
Node * estimate_expression_value(PlannerInfo *root, Node *node)
Definition: clauses.c:2286
void sampler_random_init_state(long seed, SamplerRandomState randstate)
Definition: sampling.c:229
double tuples
Definition: pathnodes.h:681
#define Min(x, y)
Definition: c.h:904
void * tsm_state
Definition: execnodes.h:1358
double sampler_random_fract(SamplerRandomState randstate)
Definition: sampling.c:238
Definition: nodes.h:525
int errcode(int sqlerrcode)
Definition: elog.c:570
uint32 BlockNumber
Definition: block.h:31
unsigned short SamplerRandomState[3]
Definition: sampling.h:20
uint16 OffsetNumber
Definition: off.h:24
PG_MODULE_MAGIC
#define linitial(l)
Definition: pg_list.h:195
NextSampleTuple_function NextSampleTuple
Definition: tsmapi.h:74
#define ERROR
Definition: elog.h:43
#define DatumGetInt64(X)
Definition: postgres.h:607
char * c
static void system_rows_samplescangetsamplesize(PlannerInfo *root, RelOptInfo *baserel, List *paramexprs, BlockNumber *pages, double *tuples)
#define FirstOffsetNumber
Definition: off.h:27
unsigned int uint32
Definition: c.h:358
NextSampleBlock_function NextSampleBlock
Definition: tsmapi.h:73
#define ereport(elevel, rest)
Definition: elog.h:141
SampleScanGetSampleSize_function SampleScanGetSampleSize
Definition: tsmapi.h:68
void * palloc0(Size size)
Definition: mcxt.c:980
uintptr_t Datum
Definition: postgres.h:367
#define list_make1_oid(x1)
Definition: pg_list.h:249
#define InvalidOffsetNumber
Definition: off.h:26
static void system_rows_initsamplescan(SampleScanState *node, int eflags)
#define makeNode(_type_)
Definition: nodes.h:573
BlockNumber pages
Definition: pathnodes.h:680
static void system_rows_beginsamplescan(SampleScanState *node, Datum *params, int nparams, uint32 seed)
#define InvalidBlockNumber
Definition: block.h:33
static OffsetNumber system_rows_nextsampletuple(SampleScanState *node, BlockNumber blockno, OffsetNumber maxoffset)
BeginSampleScan_function BeginSampleScan
Definition: tsmapi.h:72
bool repeatable_across_scans
Definition: tsmapi.h:65
PG_FUNCTION_INFO_V1(tsm_system_rows_handler)
static uint32 random_relative_prime(uint32 n, SamplerRandomState randstate)
int errmsg(const char *fmt,...)
Definition: elog.c:784
static uint32 gcd(uint32 a, uint32 b)
#define PG_FUNCTION_ARGS
Definition: fmgr.h:188
List * parameterTypes
Definition: tsmapi.h:61
#define CHECK_FOR_INTERRUPTS()
Definition: miscadmin.h:99
EndSampleScan_function EndSampleScan
Definition: tsmapi.h:75
double clamp_row_est(double nrows)
Definition: costsize.c:187
Definition: pg_list.h:50
static BlockNumber system_rows_nextsampleblock(SampleScanState *node, BlockNumber nblocks)