LCOV - code coverage report
Current view: top level - contrib/tsm_system_time - tsm_system_time.c (source / functions) Hit Total Coverage
Test: PostgreSQL 13devel Lines: 91 95 95.8 %
Date: 2019-11-13 22:07:24 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) 1996-2019, 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             : 
      37           2 : PG_MODULE_MAGIC;
      38             : 
      39           4 : 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, SamplerRandomState randstate);
      73             : 
      74             : 
      75             : /*
      76             :  * Create a TsmRoutine descriptor for the SYSTEM_TIME method.
      77             :  */
      78             : Datum
      79          82 : tsm_system_time_handler(PG_FUNCTION_ARGS)
      80             : {
      81          82 :     TsmRoutine *tsm = makeNode(TsmRoutine);
      82             : 
      83          82 :     tsm->parameterTypes = list_make1_oid(FLOAT8OID);
      84             : 
      85             :     /* See notes at head of file */
      86          82 :     tsm->repeatable_across_queries = false;
      87          82 :     tsm->repeatable_across_scans = false;
      88             : 
      89          82 :     tsm->SampleScanGetSampleSize = system_time_samplescangetsamplesize;
      90          82 :     tsm->InitSampleScan = system_time_initsamplescan;
      91          82 :     tsm->BeginSampleScan = system_time_beginsamplescan;
      92          82 :     tsm->NextSampleBlock = system_time_nextsampleblock;
      93          82 :     tsm->NextSampleTuple = system_time_nextsampletuple;
      94          82 :     tsm->EndSampleScan = NULL;
      95             : 
      96          82 :     PG_RETURN_POINTER(tsm);
      97             : }
      98             : 
      99             : /*
     100             :  * Sample size estimation.
     101             :  */
     102             : static void
     103          18 : 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          18 :     limitnode = (Node *) linitial(paramexprs);
     117          18 :     limitnode = estimate_expression_value(root, limitnode);
     118             : 
     119          32 :     if (IsA(limitnode, Const) &&
     120          14 :         !((Const *) limitnode)->constisnull)
     121             :     {
     122          14 :         millis = DatumGetFloat8(((Const *) limitnode)->constvalue);
     123          28 :         if (millis < 0 || isnan(millis))
     124             :         {
     125             :             /* Default millis if the value is bogus */
     126           4 :             millis = 1000;
     127             :         }
     128             :     }
     129             :     else
     130             :     {
     131             :         /* Default millis if we didn't obtain a non-null Const */
     132           4 :         millis = 1000;
     133             :     }
     134             : 
     135             :     /* Get the planner's idea of cost per page read */
     136          18 :     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          18 :     if (spc_random_page_cost > 0)
     147          18 :         npages = millis / spc_random_page_cost;
     148             :     else
     149           0 :         npages = millis;        /* even more bogus, but whatcha gonna do? */
     150             : 
     151             :     /* Clamp to sane value */
     152          18 :     npages = clamp_row_est(Min((double) baserel->pages, npages));
     153             : 
     154          18 :     if (baserel->tuples > 0 && baserel->pages > 0)
     155          18 :     {
     156             :         /* Estimate number of tuples returned based on tuple density */
     157          18 :         double      density = baserel->tuples / (double) baserel->pages;
     158             : 
     159          18 :         ntuples = npages * density;
     160             :     }
     161             :     else
     162             :     {
     163             :         /* For lack of data, assume one tuple per page */
     164           0 :         ntuples = npages;
     165             :     }
     166             : 
     167             :     /* Clamp to the estimated relation size */
     168          18 :     ntuples = clamp_row_est(Min(baserel->tuples, ntuples));
     169             : 
     170          18 :     *pages = npages;
     171          18 :     *tuples = ntuples;
     172          18 : }
     173             : 
     174             : /*
     175             :  * Initialize during executor setup.
     176             :  */
     177             : static void
     178          18 : system_time_initsamplescan(SampleScanState *node, int eflags)
     179             : {
     180          18 :     node->tsm_state = palloc0(sizeof(SystemTimeSamplerData));
     181             :     /* Note the above leaves tsm_state->step equal to zero */
     182          18 : }
     183             : 
     184             : /*
     185             :  * Examine parameters and prepare for a sample scan.
     186             :  */
     187             : static void
     188          12 : system_time_beginsamplescan(SampleScanState *node,
     189             :                             Datum *params,
     190             :                             int nparams,
     191             :                             uint32 seed)
     192             : {
     193          12 :     SystemTimeSamplerData *sampler = (SystemTimeSamplerData *) node->tsm_state;
     194          12 :     double      millis = DatumGetFloat8(params[0]);
     195             : 
     196          12 :     if (millis < 0 || isnan(millis))
     197           2 :         ereport(ERROR,
     198             :                 (errcode(ERRCODE_INVALID_TABLESAMPLE_ARGUMENT),
     199             :                  errmsg("sample collection time must not be negative")));
     200             : 
     201          10 :     sampler->seed = seed;
     202          10 :     sampler->millis = millis;
     203          10 :     sampler->lt = InvalidOffsetNumber;
     204          10 :     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          10 : }
     208             : 
     209             : /*
     210             :  * Select next block to sample.
     211             :  *
     212             :  * Uses linear probing algorithm for picking next block.
     213             :  */
     214             : static BlockNumber
     215          52 : system_time_nextsampleblock(SampleScanState *node, BlockNumber nblocks)
     216             : {
     217          52 :     SystemTimeSamplerData *sampler = (SystemTimeSamplerData *) node->tsm_state;
     218             :     instr_time  cur_time;
     219             : 
     220             :     /* First call within scan? */
     221          52 :     if (sampler->doneblocks == 0)
     222             :     {
     223             :         /* First scan within query? */
     224          10 :         if (sampler->step == 0)
     225             :         {
     226             :             /* Initialize now that we have scan descriptor */
     227             :             SamplerRandomState randstate;
     228             : 
     229             :             /* If relation is empty, there's nothing to scan */
     230           8 :             if (nblocks == 0)
     231           0 :                 return InvalidBlockNumber;
     232             : 
     233             :             /* We only need an RNG during this setup step */
     234           8 :             sampler_random_init_state(sampler->seed, randstate);
     235             : 
     236             :             /* Compute nblocks/firstblock/step only once per query */
     237           8 :             sampler->nblocks = nblocks;
     238             : 
     239             :             /* Choose random starting block within the relation */
     240             :             /* (Actually this is the predecessor of the first block visited) */
     241          16 :             sampler->firstblock = sampler_random_fract(randstate) *
     242           8 :                 sampler->nblocks;
     243             : 
     244             :             /* Find relative prime as step size for linear probing */
     245           8 :             sampler->step = random_relative_prime(sampler->nblocks, randstate);
     246             :         }
     247             : 
     248             :         /* Reinitialize lb and start_time */
     249          10 :         sampler->lb = sampler->firstblock;
     250          10 :         INSTR_TIME_SET_CURRENT(sampler->start_time);
     251             :     }
     252             : 
     253             :     /* If we've read all blocks in relation, we're done */
     254          52 :     if (++sampler->doneblocks > sampler->nblocks)
     255           6 :         return InvalidBlockNumber;
     256             : 
     257             :     /* If we've used up all the allotted time, we're done */
     258          46 :     INSTR_TIME_SET_CURRENT(cur_time);
     259          46 :     INSTR_TIME_SUBTRACT(cur_time, sampler->start_time);
     260          46 :     if (INSTR_TIME_GET_MILLISEC(cur_time) >= sampler->millis)
     261           4 :         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          42 :         sampler->lb = ((uint64) sampler->lb + sampler->step) % sampler->nblocks;
     273          42 :     } while (sampler->lb >= nblocks);
     274             : 
     275          42 :     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         228 : system_time_nextsampletuple(SampleScanState *node,
     289             :                             BlockNumber blockno,
     290             :                             OffsetNumber maxoffset)
     291             : {
     292         228 :     SystemTimeSamplerData *sampler = (SystemTimeSamplerData *) node->tsm_state;
     293         228 :     OffsetNumber tupoffset = sampler->lt;
     294             : 
     295             :     /* Advance to next possible offset on page */
     296         228 :     if (tupoffset == InvalidOffsetNumber)
     297          42 :         tupoffset = FirstOffsetNumber;
     298             :     else
     299         186 :         tupoffset++;
     300             : 
     301             :     /* Done? */
     302         228 :     if (tupoffset > maxoffset)
     303          42 :         tupoffset = InvalidOffsetNumber;
     304             : 
     305         228 :     sampler->lt = tupoffset;
     306             : 
     307         228 :     return tupoffset;
     308             : }
     309             : 
     310             : /*
     311             :  * Compute greatest common divisor of two uint32's.
     312             :  */
     313             : static uint32
     314           8 : gcd(uint32 a, uint32 b)
     315             : {
     316             :     uint32      c;
     317             : 
     318          30 :     while (a != 0)
     319             :     {
     320          14 :         c = a;
     321          14 :         a = b % a;
     322          14 :         b = c;
     323             :     }
     324             : 
     325           8 :     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
     333           8 : random_relative_prime(uint32 n, SamplerRandomState randstate)
     334             : {
     335             :     uint32      r;
     336             : 
     337             :     /* Safety check to avoid infinite loop or zero result for small n. */
     338           8 :     if (n <= 1)
     339           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             :     {
     348           8 :         CHECK_FOR_INTERRUPTS();
     349           8 :         r = (uint32) (sampler_random_fract(randstate) * n);
     350           8 :     } while (r == 0 || gcd(r, n) > 1);
     351             : 
     352           8 :     return r;
     353             : }

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