LCOV - code coverage report
Current view: top level - contrib/tsm_system_rows - tsm_system_rows.c (source / functions) Hit Total Coverage
Test: PostgreSQL 18devel Lines: 89 92 96.7 %
Date: 2025-04-01 15:15:16 Functions: 10 10 100.0 %
Legend: Lines: hit not hit

          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) 1996-2025, 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/tsmapi.h"
      32             : #include "catalog/pg_type.h"
      33             : #include "miscadmin.h"
      34             : #include "optimizer/optimizer.h"
      35             : #include "utils/sampling.h"
      36             : 
      37           2 : PG_MODULE_MAGIC_EXT(
      38             :                     .name = "tsm_system_rows",
      39             :                     .version = PG_VERSION
      40             : );
      41             : 
      42           4 : PG_FUNCTION_INFO_V1(tsm_system_rows_handler);
      43             : 
      44             : 
      45             : /* Private state */
      46             : typedef struct
      47             : {
      48             :     uint32      seed;           /* random seed */
      49             :     int64       ntuples;        /* number of tuples to return */
      50             :     OffsetNumber lt;            /* last tuple returned from current block */
      51             :     BlockNumber doneblocks;     /* number of already-scanned blocks */
      52             :     BlockNumber lb;             /* last block visited */
      53             :     /* these three values are not changed during a rescan: */
      54             :     BlockNumber nblocks;        /* number of blocks in relation */
      55             :     BlockNumber firstblock;     /* first block to sample from */
      56             :     BlockNumber step;           /* step size, or 0 if not set yet */
      57             : } SystemRowsSamplerData;
      58             : 
      59             : static void system_rows_samplescangetsamplesize(PlannerInfo *root,
      60             :                                                 RelOptInfo *baserel,
      61             :                                                 List *paramexprs,
      62             :                                                 BlockNumber *pages,
      63             :                                                 double *tuples);
      64             : static void system_rows_initsamplescan(SampleScanState *node,
      65             :                                        int eflags);
      66             : static void system_rows_beginsamplescan(SampleScanState *node,
      67             :                                         Datum *params,
      68             :                                         int nparams,
      69             :                                         uint32 seed);
      70             : static BlockNumber system_rows_nextsampleblock(SampleScanState *node, BlockNumber nblocks);
      71             : static OffsetNumber system_rows_nextsampletuple(SampleScanState *node,
      72             :                                                 BlockNumber blockno,
      73             :                                                 OffsetNumber maxoffset);
      74             : static uint32 random_relative_prime(uint32 n, pg_prng_state *randstate);
      75             : 
      76             : 
      77             : /*
      78             :  * Create a TsmRoutine descriptor for the SYSTEM_ROWS method.
      79             :  */
      80             : Datum
      81          80 : tsm_system_rows_handler(PG_FUNCTION_ARGS)
      82             : {
      83          80 :     TsmRoutine *tsm = makeNode(TsmRoutine);
      84             : 
      85          80 :     tsm->parameterTypes = list_make1_oid(INT8OID);
      86             : 
      87             :     /* See notes at head of file */
      88          80 :     tsm->repeatable_across_queries = false;
      89          80 :     tsm->repeatable_across_scans = true;
      90             : 
      91          80 :     tsm->SampleScanGetSampleSize = system_rows_samplescangetsamplesize;
      92          80 :     tsm->InitSampleScan = system_rows_initsamplescan;
      93          80 :     tsm->BeginSampleScan = system_rows_beginsamplescan;
      94          80 :     tsm->NextSampleBlock = system_rows_nextsampleblock;
      95          80 :     tsm->NextSampleTuple = system_rows_nextsampletuple;
      96          80 :     tsm->EndSampleScan = NULL;
      97             : 
      98          80 :     PG_RETURN_POINTER(tsm);
      99             : }
     100             : 
     101             : /*
     102             :  * Sample size estimation.
     103             :  */
     104             : static void
     105          18 : system_rows_samplescangetsamplesize(PlannerInfo *root,
     106             :                                     RelOptInfo *baserel,
     107             :                                     List *paramexprs,
     108             :                                     BlockNumber *pages,
     109             :                                     double *tuples)
     110             : {
     111             :     Node       *limitnode;
     112             :     int64       ntuples;
     113             :     double      npages;
     114             : 
     115             :     /* Try to extract an estimate for the limit rowcount */
     116          18 :     limitnode = (Node *) linitial(paramexprs);
     117          18 :     limitnode = estimate_expression_value(root, limitnode);
     118             : 
     119          18 :     if (IsA(limitnode, Const) &&
     120          14 :         !((Const *) limitnode)->constisnull)
     121             :     {
     122          14 :         ntuples = DatumGetInt64(((Const *) limitnode)->constvalue);
     123          14 :         if (ntuples < 0)
     124             :         {
     125             :             /* Default ntuples if the value is bogus */
     126           4 :             ntuples = 1000;
     127             :         }
     128             :     }
     129             :     else
     130             :     {
     131             :         /* Default ntuples if we didn't obtain a non-null Const */
     132           4 :         ntuples = 1000;
     133             :     }
     134             : 
     135             :     /* Clamp to the estimated relation size */
     136          18 :     if (ntuples > baserel->tuples)
     137          10 :         ntuples = (int64) baserel->tuples;
     138          18 :     ntuples = clamp_row_est(ntuples);
     139             : 
     140          18 :     if (baserel->tuples > 0 && baserel->pages > 0)
     141          18 :     {
     142             :         /* Estimate number of pages visited based on tuple density */
     143          18 :         double      density = baserel->tuples / (double) baserel->pages;
     144             : 
     145          18 :         npages = ntuples / density;
     146             :     }
     147             :     else
     148             :     {
     149             :         /* For lack of data, assume one tuple per page */
     150           0 :         npages = ntuples;
     151             :     }
     152             : 
     153             :     /* Clamp to sane value */
     154          18 :     npages = clamp_row_est(Min((double) baserel->pages, npages));
     155             : 
     156          18 :     *pages = npages;
     157          18 :     *tuples = ntuples;
     158          18 : }
     159             : 
     160             : /*
     161             :  * Initialize during executor setup.
     162             :  */
     163             : static void
     164          18 : system_rows_initsamplescan(SampleScanState *node, int eflags)
     165             : {
     166          18 :     node->tsm_state = palloc0(sizeof(SystemRowsSamplerData));
     167             :     /* Note the above leaves tsm_state->step equal to zero */
     168          18 : }
     169             : 
     170             : /*
     171             :  * Examine parameters and prepare for a sample scan.
     172             :  */
     173             : static void
     174          18 : system_rows_beginsamplescan(SampleScanState *node,
     175             :                             Datum *params,
     176             :                             int nparams,
     177             :                             uint32 seed)
     178             : {
     179          18 :     SystemRowsSamplerData *sampler = (SystemRowsSamplerData *) node->tsm_state;
     180          18 :     int64       ntuples = DatumGetInt64(params[0]);
     181             : 
     182          18 :     if (ntuples < 0)
     183           2 :         ereport(ERROR,
     184             :                 (errcode(ERRCODE_INVALID_TABLESAMPLE_ARGUMENT),
     185             :                  errmsg("sample size must not be negative")));
     186             : 
     187          16 :     sampler->seed = seed;
     188          16 :     sampler->ntuples = ntuples;
     189          16 :     sampler->lt = InvalidOffsetNumber;
     190          16 :     sampler->doneblocks = 0;
     191             :     /* lb will be initialized during first NextSampleBlock call */
     192             :     /* we intentionally do not change nblocks/firstblock/step here */
     193             : 
     194             :     /*
     195             :      * We *must* use pagemode visibility checking in this module, so force
     196             :      * that even though it's currently default.
     197             :      */
     198          16 :     node->use_pagemode = true;
     199          16 : }
     200             : 
     201             : /*
     202             :  * Select next block to sample.
     203             :  *
     204             :  * Uses linear probing algorithm for picking next block.
     205             :  */
     206             : static BlockNumber
     207          64 : system_rows_nextsampleblock(SampleScanState *node, BlockNumber nblocks)
     208             : {
     209          64 :     SystemRowsSamplerData *sampler = (SystemRowsSamplerData *) node->tsm_state;
     210             : 
     211             :     /* First call within scan? */
     212          64 :     if (sampler->doneblocks == 0)
     213             :     {
     214             :         /* First scan within query? */
     215          16 :         if (sampler->step == 0)
     216             :         {
     217             :             /* Initialize now that we have scan descriptor */
     218             :             pg_prng_state randstate;
     219             : 
     220             :             /* If relation is empty, there's nothing to scan */
     221          12 :             if (nblocks == 0)
     222           0 :                 return InvalidBlockNumber;
     223             : 
     224             :             /* We only need an RNG during this setup step */
     225          12 :             sampler_random_init_state(sampler->seed, &randstate);
     226             : 
     227             :             /* Compute nblocks/firstblock/step only once per query */
     228          12 :             sampler->nblocks = nblocks;
     229             : 
     230             :             /* Choose random starting block within the relation */
     231             :             /* (Actually this is the predecessor of the first block visited) */
     232          12 :             sampler->firstblock = sampler_random_fract(&randstate) *
     233          12 :                 sampler->nblocks;
     234             : 
     235             :             /* Find relative prime as step size for linear probing */
     236          12 :             sampler->step = random_relative_prime(sampler->nblocks, &randstate);
     237             :         }
     238             : 
     239             :         /* Reinitialize lb */
     240          16 :         sampler->lb = sampler->firstblock;
     241             :     }
     242             : 
     243             :     /* If we've read all blocks or returned all needed tuples, we're done */
     244          64 :     if (++sampler->doneblocks > sampler->nblocks ||
     245          60 :         node->donetuples >= sampler->ntuples)
     246          16 :         return InvalidBlockNumber;
     247             : 
     248             :     /*
     249             :      * It's probably impossible for scan->rs_nblocks to decrease between scans
     250             :      * within a query; but just in case, loop until we select a block number
     251             :      * less than scan->rs_nblocks.  We don't care if scan->rs_nblocks has
     252             :      * increased since the first scan.
     253             :      */
     254             :     do
     255             :     {
     256             :         /* Advance lb, using uint64 arithmetic to forestall overflow */
     257          48 :         sampler->lb = ((uint64) sampler->lb + sampler->step) % sampler->nblocks;
     258          48 :     } while (sampler->lb >= nblocks);
     259             : 
     260          48 :     return sampler->lb;
     261             : }
     262             : 
     263             : /*
     264             :  * Select next sampled tuple in current block.
     265             :  *
     266             :  * In block sampling, we just want to sample all the tuples in each selected
     267             :  * block.
     268             :  *
     269             :  * When we reach end of the block, return InvalidOffsetNumber which tells
     270             :  * SampleScan to go to next block.
     271             :  */
     272             : static OffsetNumber
     273         254 : system_rows_nextsampletuple(SampleScanState *node,
     274             :                             BlockNumber blockno,
     275             :                             OffsetNumber maxoffset)
     276             : {
     277         254 :     SystemRowsSamplerData *sampler = (SystemRowsSamplerData *) node->tsm_state;
     278         254 :     OffsetNumber tupoffset = sampler->lt;
     279             : 
     280             :     /* Quit if we've returned all needed tuples */
     281         254 :     if (node->donetuples >= sampler->ntuples)
     282           8 :         return InvalidOffsetNumber;
     283             : 
     284             :     /* Advance to next possible offset on page */
     285         246 :     if (tupoffset == InvalidOffsetNumber)
     286          48 :         tupoffset = FirstOffsetNumber;
     287             :     else
     288         198 :         tupoffset++;
     289             : 
     290             :     /* Done? */
     291         246 :     if (tupoffset > maxoffset)
     292          40 :         tupoffset = InvalidOffsetNumber;
     293             : 
     294         246 :     sampler->lt = tupoffset;
     295             : 
     296         246 :     return tupoffset;
     297             : }
     298             : 
     299             : /*
     300             :  * Compute greatest common divisor of two uint32's.
     301             :  */
     302             : static uint32
     303          12 : gcd(uint32 a, uint32 b)
     304             : {
     305             :     uint32      c;
     306             : 
     307          36 :     while (a != 0)
     308             :     {
     309          24 :         c = a;
     310          24 :         a = b % a;
     311          24 :         b = c;
     312             :     }
     313             : 
     314          12 :     return b;
     315             : }
     316             : 
     317             : /*
     318             :  * Pick a random value less than and relatively prime to n, if possible
     319             :  * (else return 1).
     320             :  */
     321             : static uint32
     322          12 : random_relative_prime(uint32 n, pg_prng_state *randstate)
     323             : {
     324             :     uint32      r;
     325             : 
     326             :     /* Safety check to avoid infinite loop or zero result for small n. */
     327          12 :     if (n <= 1)
     328           0 :         return 1;
     329             : 
     330             :     /*
     331             :      * This should only take 2 or 3 iterations as the probability of 2 numbers
     332             :      * being relatively prime is ~61%; but just in case, we'll include a
     333             :      * CHECK_FOR_INTERRUPTS in the loop.
     334             :      */
     335             :     do
     336             :     {
     337          18 :         CHECK_FOR_INTERRUPTS();
     338          18 :         r = (uint32) (sampler_random_fract(randstate) * n);
     339          18 :     } while (r == 0 || gcd(r, n) > 1);
     340             : 
     341          12 :     return r;
     342             : }

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