LCOV - code coverage report
Current view: top level - contrib/tsm_system_rows - tsm_system_rows.c (source / functions) Hit Total Coverage
Test: PostgreSQL 12beta2 Lines: 89 92 96.7 %
Date: 2019-06-18 07:06:57 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) 1996-2019, 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             : 
      38           2 : PG_MODULE_MAGIC;
      39             : 
      40           4 : 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, SamplerRandomState randstate);
      73             : 
      74             : 
      75             : /*
      76             :  * Create a TsmRoutine descriptor for the SYSTEM_ROWS method.
      77             :  */
      78             : Datum
      79          80 : tsm_system_rows_handler(PG_FUNCTION_ARGS)
      80             : {
      81          80 :     TsmRoutine *tsm = makeNode(TsmRoutine);
      82             : 
      83          80 :     tsm->parameterTypes = list_make1_oid(INT8OID);
      84             : 
      85             :     /* See notes at head of file */
      86          80 :     tsm->repeatable_across_queries = false;
      87          80 :     tsm->repeatable_across_scans = true;
      88             : 
      89          80 :     tsm->SampleScanGetSampleSize = system_rows_samplescangetsamplesize;
      90          80 :     tsm->InitSampleScan = system_rows_initsamplescan;
      91          80 :     tsm->BeginSampleScan = system_rows_beginsamplescan;
      92          80 :     tsm->NextSampleBlock = system_rows_nextsampleblock;
      93          80 :     tsm->NextSampleTuple = system_rows_nextsampletuple;
      94          80 :     tsm->EndSampleScan = NULL;
      95             : 
      96          80 :     PG_RETURN_POINTER(tsm);
      97             : }
      98             : 
      99             : /*
     100             :  * Sample size estimation.
     101             :  */
     102             : static void
     103          18 : 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          18 :     limitnode = (Node *) linitial(paramexprs);
     115          18 :     limitnode = estimate_expression_value(root, limitnode);
     116             : 
     117          32 :     if (IsA(limitnode, Const) &&
     118          14 :         !((Const *) limitnode)->constisnull)
     119             :     {
     120          14 :         ntuples = DatumGetInt64(((Const *) limitnode)->constvalue);
     121          28 :         if (ntuples < 0)
     122             :         {
     123             :             /* Default ntuples if the value is bogus */
     124           4 :             ntuples = 1000;
     125             :         }
     126             :     }
     127             :     else
     128             :     {
     129             :         /* Default ntuples if we didn't obtain a non-null Const */
     130           4 :         ntuples = 1000;
     131             :     }
     132             : 
     133             :     /* Clamp to the estimated relation size */
     134          18 :     if (ntuples > baserel->tuples)
     135          10 :         ntuples = (int64) baserel->tuples;
     136          18 :     ntuples = clamp_row_est(ntuples);
     137             : 
     138          18 :     if (baserel->tuples > 0 && baserel->pages > 0)
     139          18 :     {
     140             :         /* Estimate number of pages visited based on tuple density */
     141          18 :         double      density = baserel->tuples / (double) baserel->pages;
     142             : 
     143          18 :         npages = ntuples / density;
     144             :     }
     145             :     else
     146             :     {
     147             :         /* For lack of data, assume one tuple per page */
     148           0 :         npages = ntuples;
     149             :     }
     150             : 
     151             :     /* Clamp to sane value */
     152          18 :     npages = clamp_row_est(Min((double) baserel->pages, npages));
     153             : 
     154          18 :     *pages = npages;
     155          18 :     *tuples = ntuples;
     156          18 : }
     157             : 
     158             : /*
     159             :  * Initialize during executor setup.
     160             :  */
     161             : static void
     162          18 : system_rows_initsamplescan(SampleScanState *node, int eflags)
     163             : {
     164          18 :     node->tsm_state = palloc0(sizeof(SystemRowsSamplerData));
     165             :     /* Note the above leaves tsm_state->step equal to zero */
     166          18 : }
     167             : 
     168             : /*
     169             :  * Examine parameters and prepare for a sample scan.
     170             :  */
     171             : static void
     172          18 : system_rows_beginsamplescan(SampleScanState *node,
     173             :                             Datum *params,
     174             :                             int nparams,
     175             :                             uint32 seed)
     176             : {
     177          18 :     SystemRowsSamplerData *sampler = (SystemRowsSamplerData *) node->tsm_state;
     178          18 :     int64       ntuples = DatumGetInt64(params[0]);
     179             : 
     180          18 :     if (ntuples < 0)
     181           2 :         ereport(ERROR,
     182             :                 (errcode(ERRCODE_INVALID_TABLESAMPLE_ARGUMENT),
     183             :                  errmsg("sample size must not be negative")));
     184             : 
     185          16 :     sampler->seed = seed;
     186          16 :     sampler->ntuples = ntuples;
     187          16 :     sampler->lt = InvalidOffsetNumber;
     188          16 :     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          16 :     node->use_pagemode = true;
     197          16 : }
     198             : 
     199             : /*
     200             :  * Select next block to sample.
     201             :  *
     202             :  * Uses linear probing algorithm for picking next block.
     203             :  */
     204             : static BlockNumber
     205          66 : system_rows_nextsampleblock(SampleScanState *node, BlockNumber nblocks)
     206             : {
     207          66 :     SystemRowsSamplerData *sampler = (SystemRowsSamplerData *) node->tsm_state;
     208             : 
     209             :     /* First call within scan? */
     210          66 :     if (sampler->doneblocks == 0)
     211             :     {
     212             :         /* First scan within query? */
     213          16 :         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          12 :             if (nblocks == 0)
     220           0 :                 return InvalidBlockNumber;
     221             : 
     222             :             /* We only need an RNG during this setup step */
     223          12 :             sampler_random_init_state(sampler->seed, randstate);
     224             : 
     225             :             /* Compute nblocks/firstblock/step only once per query */
     226          12 :             sampler->nblocks = nblocks;
     227             : 
     228             :             /* Choose random starting block within the relation */
     229             :             /* (Actually this is the predecessor of the first block visited) */
     230          24 :             sampler->firstblock = sampler_random_fract(randstate) *
     231          12 :                 sampler->nblocks;
     232             : 
     233             :             /* Find relative prime as step size for linear probing */
     234          12 :             sampler->step = random_relative_prime(sampler->nblocks, randstate);
     235             :         }
     236             : 
     237             :         /* Reinitialize lb */
     238          16 :         sampler->lb = sampler->firstblock;
     239             :     }
     240             : 
     241             :     /* If we've read all blocks or returned all needed tuples, we're done */
     242         128 :     if (++sampler->doneblocks > sampler->nblocks ||
     243          62 :         node->donetuples >= sampler->ntuples)
     244          16 :         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          50 :         sampler->lb = ((uint64) sampler->lb + sampler->step) % sampler->nblocks;
     256          50 :     } while (sampler->lb >= nblocks);
     257             : 
     258          50 :     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         256 : system_rows_nextsampletuple(SampleScanState *node,
     272             :                             BlockNumber blockno,
     273             :                             OffsetNumber maxoffset)
     274             : {
     275         256 :     SystemRowsSamplerData *sampler = (SystemRowsSamplerData *) node->tsm_state;
     276         256 :     OffsetNumber tupoffset = sampler->lt;
     277             : 
     278             :     /* Quit if we've returned all needed tuples */
     279         256 :     if (node->donetuples >= sampler->ntuples)
     280           8 :         return InvalidOffsetNumber;
     281             : 
     282             :     /* Advance to next possible offset on page */
     283         248 :     if (tupoffset == InvalidOffsetNumber)
     284          50 :         tupoffset = FirstOffsetNumber;
     285             :     else
     286         198 :         tupoffset++;
     287             : 
     288             :     /* Done? */
     289         248 :     if (tupoffset > maxoffset)
     290          42 :         tupoffset = InvalidOffsetNumber;
     291             : 
     292         248 :     sampler->lt = tupoffset;
     293             : 
     294         248 :     return tupoffset;
     295             : }
     296             : 
     297             : /*
     298             :  * Compute greatest common divisor of two uint32's.
     299             :  */
     300             : static uint32
     301          12 : gcd(uint32 a, uint32 b)
     302             : {
     303             :     uint32      c;
     304             : 
     305          52 :     while (a != 0)
     306             :     {
     307          28 :         c = a;
     308          28 :         a = b % a;
     309          28 :         b = c;
     310             :     }
     311             : 
     312          12 :     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
     320          12 : random_relative_prime(uint32 n, SamplerRandomState randstate)
     321             : {
     322             :     uint32      r;
     323             : 
     324             :     /* Safety check to avoid infinite loop or zero result for small n. */
     325          12 :     if (n <= 1)
     326           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             :     {
     335          18 :         CHECK_FOR_INTERRUPTS();
     336          18 :         r = (uint32) (sampler_random_fract(randstate) * n);
     337          18 :     } while (r == 0 || gcd(r, n) > 1);
     338             : 
     339          12 :     return r;
     340             : }

Generated by: LCOV version 1.13