LCOV - code coverage report
Current view: top level - contrib/tsm_system_time - tsm_system_time.c (source / functions) Hit Total Coverage
Test: PostgreSQL 18devel Lines: 91 95 95.8 %
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_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) 1996-2025, 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/tsmapi.h"
      30             : #include "catalog/pg_type.h"
      31             : #include "miscadmin.h"
      32             : #include "optimizer/optimizer.h"
      33             : #include "utils/sampling.h"
      34             : #include "utils/spccache.h"
      35             : 
      36           2 : PG_MODULE_MAGIC_EXT(
      37             :                     .name = "tsm_system_time",
      38             :                     .version = PG_VERSION
      39             : );
      40             : 
      41           4 : PG_FUNCTION_INFO_V1(tsm_system_time_handler);
      42             : 
      43             : 
      44             : /* Private state */
      45             : typedef struct
      46             : {
      47             :     uint32      seed;           /* random seed */
      48             :     double      millis;         /* time limit for sampling */
      49             :     instr_time  start_time;     /* scan start time */
      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             : } SystemTimeSamplerData;
      58             : 
      59             : static void system_time_samplescangetsamplesize(PlannerInfo *root,
      60             :                                                 RelOptInfo *baserel,
      61             :                                                 List *paramexprs,
      62             :                                                 BlockNumber *pages,
      63             :                                                 double *tuples);
      64             : static void system_time_initsamplescan(SampleScanState *node,
      65             :                                        int eflags);
      66             : static void system_time_beginsamplescan(SampleScanState *node,
      67             :                                         Datum *params,
      68             :                                         int nparams,
      69             :                                         uint32 seed);
      70             : static BlockNumber system_time_nextsampleblock(SampleScanState *node, BlockNumber nblocks);
      71             : static OffsetNumber system_time_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_TIME method.
      79             :  */
      80             : Datum
      81          82 : tsm_system_time_handler(PG_FUNCTION_ARGS)
      82             : {
      83          82 :     TsmRoutine *tsm = makeNode(TsmRoutine);
      84             : 
      85          82 :     tsm->parameterTypes = list_make1_oid(FLOAT8OID);
      86             : 
      87             :     /* See notes at head of file */
      88          82 :     tsm->repeatable_across_queries = false;
      89          82 :     tsm->repeatable_across_scans = false;
      90             : 
      91          82 :     tsm->SampleScanGetSampleSize = system_time_samplescangetsamplesize;
      92          82 :     tsm->InitSampleScan = system_time_initsamplescan;
      93          82 :     tsm->BeginSampleScan = system_time_beginsamplescan;
      94          82 :     tsm->NextSampleBlock = system_time_nextsampleblock;
      95          82 :     tsm->NextSampleTuple = system_time_nextsampletuple;
      96          82 :     tsm->EndSampleScan = NULL;
      97             : 
      98          82 :     PG_RETURN_POINTER(tsm);
      99             : }
     100             : 
     101             : /*
     102             :  * Sample size estimation.
     103             :  */
     104             : static void
     105          18 : system_time_samplescangetsamplesize(PlannerInfo *root,
     106             :                                     RelOptInfo *baserel,
     107             :                                     List *paramexprs,
     108             :                                     BlockNumber *pages,
     109             :                                     double *tuples)
     110             : {
     111             :     Node       *limitnode;
     112             :     double      millis;
     113             :     double      spc_random_page_cost;
     114             :     double      npages;
     115             :     double      ntuples;
     116             : 
     117             :     /* Try to extract an estimate for the limit time spec */
     118          18 :     limitnode = (Node *) linitial(paramexprs);
     119          18 :     limitnode = estimate_expression_value(root, limitnode);
     120             : 
     121          18 :     if (IsA(limitnode, Const) &&
     122          14 :         !((Const *) limitnode)->constisnull)
     123             :     {
     124          14 :         millis = DatumGetFloat8(((Const *) limitnode)->constvalue);
     125          14 :         if (millis < 0 || isnan(millis))
     126             :         {
     127             :             /* Default millis if the value is bogus */
     128           4 :             millis = 1000;
     129             :         }
     130             :     }
     131             :     else
     132             :     {
     133             :         /* Default millis if we didn't obtain a non-null Const */
     134           4 :         millis = 1000;
     135             :     }
     136             : 
     137             :     /* Get the planner's idea of cost per page read */
     138          18 :     get_tablespace_page_costs(baserel->reltablespace,
     139             :                               &spc_random_page_cost,
     140             :                               NULL);
     141             : 
     142             :     /*
     143             :      * Estimate the number of pages we can read by assuming that the cost
     144             :      * figure is expressed in milliseconds.  This is completely, unmistakably
     145             :      * bogus, but we have to do something to produce an estimate and there's
     146             :      * no better answer.
     147             :      */
     148          18 :     if (spc_random_page_cost > 0)
     149          18 :         npages = millis / spc_random_page_cost;
     150             :     else
     151           0 :         npages = millis;        /* even more bogus, but whatcha gonna do? */
     152             : 
     153             :     /* Clamp to sane value */
     154          18 :     npages = clamp_row_est(Min((double) baserel->pages, npages));
     155             : 
     156          18 :     if (baserel->tuples > 0 && baserel->pages > 0)
     157          18 :     {
     158             :         /* Estimate number of tuples returned based on tuple density */
     159          18 :         double      density = baserel->tuples / (double) baserel->pages;
     160             : 
     161          18 :         ntuples = npages * density;
     162             :     }
     163             :     else
     164             :     {
     165             :         /* For lack of data, assume one tuple per page */
     166           0 :         ntuples = npages;
     167             :     }
     168             : 
     169             :     /* Clamp to the estimated relation size */
     170          18 :     ntuples = clamp_row_est(Min(baserel->tuples, ntuples));
     171             : 
     172          18 :     *pages = npages;
     173          18 :     *tuples = ntuples;
     174          18 : }
     175             : 
     176             : /*
     177             :  * Initialize during executor setup.
     178             :  */
     179             : static void
     180          18 : system_time_initsamplescan(SampleScanState *node, int eflags)
     181             : {
     182          18 :     node->tsm_state = palloc0(sizeof(SystemTimeSamplerData));
     183             :     /* Note the above leaves tsm_state->step equal to zero */
     184          18 : }
     185             : 
     186             : /*
     187             :  * Examine parameters and prepare for a sample scan.
     188             :  */
     189             : static void
     190          12 : system_time_beginsamplescan(SampleScanState *node,
     191             :                             Datum *params,
     192             :                             int nparams,
     193             :                             uint32 seed)
     194             : {
     195          12 :     SystemTimeSamplerData *sampler = (SystemTimeSamplerData *) node->tsm_state;
     196          12 :     double      millis = DatumGetFloat8(params[0]);
     197             : 
     198          12 :     if (millis < 0 || isnan(millis))
     199           2 :         ereport(ERROR,
     200             :                 (errcode(ERRCODE_INVALID_TABLESAMPLE_ARGUMENT),
     201             :                  errmsg("sample collection time must not be negative")));
     202             : 
     203          10 :     sampler->seed = seed;
     204          10 :     sampler->millis = millis;
     205          10 :     sampler->lt = InvalidOffsetNumber;
     206          10 :     sampler->doneblocks = 0;
     207             :     /* start_time, lb will be initialized during first NextSampleBlock call */
     208             :     /* we intentionally do not change nblocks/firstblock/step here */
     209          10 : }
     210             : 
     211             : /*
     212             :  * Select next block to sample.
     213             :  *
     214             :  * Uses linear probing algorithm for picking next block.
     215             :  */
     216             : static BlockNumber
     217          52 : system_time_nextsampleblock(SampleScanState *node, BlockNumber nblocks)
     218             : {
     219          52 :     SystemTimeSamplerData *sampler = (SystemTimeSamplerData *) node->tsm_state;
     220             :     instr_time  cur_time;
     221             : 
     222             :     /* First call within scan? */
     223          52 :     if (sampler->doneblocks == 0)
     224             :     {
     225             :         /* First scan within query? */
     226          10 :         if (sampler->step == 0)
     227             :         {
     228             :             /* Initialize now that we have scan descriptor */
     229             :             pg_prng_state randstate;
     230             : 
     231             :             /* If relation is empty, there's nothing to scan */
     232           8 :             if (nblocks == 0)
     233           0 :                 return InvalidBlockNumber;
     234             : 
     235             :             /* We only need an RNG during this setup step */
     236           8 :             sampler_random_init_state(sampler->seed, &randstate);
     237             : 
     238             :             /* Compute nblocks/firstblock/step only once per query */
     239           8 :             sampler->nblocks = nblocks;
     240             : 
     241             :             /* Choose random starting block within the relation */
     242             :             /* (Actually this is the predecessor of the first block visited) */
     243           8 :             sampler->firstblock = sampler_random_fract(&randstate) *
     244           8 :                 sampler->nblocks;
     245             : 
     246             :             /* Find relative prime as step size for linear probing */
     247           8 :             sampler->step = random_relative_prime(sampler->nblocks, &randstate);
     248             :         }
     249             : 
     250             :         /* Reinitialize lb and start_time */
     251          10 :         sampler->lb = sampler->firstblock;
     252          10 :         INSTR_TIME_SET_CURRENT(sampler->start_time);
     253             :     }
     254             : 
     255             :     /* If we've read all blocks in relation, we're done */
     256          52 :     if (++sampler->doneblocks > sampler->nblocks)
     257           6 :         return InvalidBlockNumber;
     258             : 
     259             :     /* If we've used up all the allotted time, we're done */
     260          46 :     INSTR_TIME_SET_CURRENT(cur_time);
     261          46 :     INSTR_TIME_SUBTRACT(cur_time, sampler->start_time);
     262          46 :     if (INSTR_TIME_GET_MILLISEC(cur_time) >= sampler->millis)
     263           4 :         return InvalidBlockNumber;
     264             : 
     265             :     /*
     266             :      * It's probably impossible for scan->rs_nblocks to decrease between scans
     267             :      * within a query; but just in case, loop until we select a block number
     268             :      * less than scan->rs_nblocks.  We don't care if scan->rs_nblocks has
     269             :      * increased since the first scan.
     270             :      */
     271             :     do
     272             :     {
     273             :         /* Advance lb, using uint64 arithmetic to forestall overflow */
     274          42 :         sampler->lb = ((uint64) sampler->lb + sampler->step) % sampler->nblocks;
     275          42 :     } while (sampler->lb >= nblocks);
     276             : 
     277          42 :     return sampler->lb;
     278             : }
     279             : 
     280             : /*
     281             :  * Select next sampled tuple in current block.
     282             :  *
     283             :  * In block sampling, we just want to sample all the tuples in each selected
     284             :  * block.
     285             :  *
     286             :  * When we reach end of the block, return InvalidOffsetNumber which tells
     287             :  * SampleScan to go to next block.
     288             :  */
     289             : static OffsetNumber
     290         228 : system_time_nextsampletuple(SampleScanState *node,
     291             :                             BlockNumber blockno,
     292             :                             OffsetNumber maxoffset)
     293             : {
     294         228 :     SystemTimeSamplerData *sampler = (SystemTimeSamplerData *) node->tsm_state;
     295         228 :     OffsetNumber tupoffset = sampler->lt;
     296             : 
     297             :     /* Advance to next possible offset on page */
     298         228 :     if (tupoffset == InvalidOffsetNumber)
     299          42 :         tupoffset = FirstOffsetNumber;
     300             :     else
     301         186 :         tupoffset++;
     302             : 
     303             :     /* Done? */
     304         228 :     if (tupoffset > maxoffset)
     305          42 :         tupoffset = InvalidOffsetNumber;
     306             : 
     307         228 :     sampler->lt = tupoffset;
     308             : 
     309         228 :     return tupoffset;
     310             : }
     311             : 
     312             : /*
     313             :  * Compute greatest common divisor of two uint32's.
     314             :  */
     315             : static uint32
     316           8 : gcd(uint32 a, uint32 b)
     317             : {
     318             :     uint32      c;
     319             : 
     320          30 :     while (a != 0)
     321             :     {
     322          22 :         c = a;
     323          22 :         a = b % a;
     324          22 :         b = c;
     325             :     }
     326             : 
     327           8 :     return b;
     328             : }
     329             : 
     330             : /*
     331             :  * Pick a random value less than and relatively prime to n, if possible
     332             :  * (else return 1).
     333             :  */
     334             : static uint32
     335           8 : random_relative_prime(uint32 n, pg_prng_state *randstate)
     336             : {
     337             :     uint32      r;
     338             : 
     339             :     /* Safety check to avoid infinite loop or zero result for small n. */
     340           8 :     if (n <= 1)
     341           0 :         return 1;
     342             : 
     343             :     /*
     344             :      * This should only take 2 or 3 iterations as the probability of 2 numbers
     345             :      * being relatively prime is ~61%; but just in case, we'll include a
     346             :      * CHECK_FOR_INTERRUPTS in the loop.
     347             :      */
     348             :     do
     349             :     {
     350          10 :         CHECK_FOR_INTERRUPTS();
     351          10 :         r = (uint32) (sampler_random_fract(randstate) * n);
     352          10 :     } while (r == 0 || gcd(r, n) > 1);
     353             : 
     354           8 :     return r;
     355             : }

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