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  • PostgreSQL在何处处理 sql查询之六十五

    看hash join 里的概念:

    numbuckets与numbatches:

    numbuckets是 在内存中,hash表里面,"桶"的个数。

    numbatches是 如果hash表过大,在内存中放不下,则要分多次进行。

    void
    final_cost_hashjoin(PlannerInfo *root, HashPath *path,
                        JoinCostWorkspace *workspace,
                        SpecialJoinInfo *sjinfo,
                        SemiAntiJoinFactors *semifactors)
    {
        ...
        /* and compute the number of "virtual" buckets in the whole join */
        virtualbuckets = (double) numbuckets *(double) numbatches;
        ...
    }

    http://www.postgresql.org/docs/current/static/using-explain.html

    EXPLAIN ANALYZE SELECT *
    FROM tenk1 t1, tenk2 t2
    WHERE t1.unique1 < 100 AND t1.unique2 = t2.unique2 ORDER BY t1.fivethous;
    
                                                                     QUERY PLAN
    --------------------------------------------------------------------------------------------------------------------------------------------
     Sort  (cost=717.30..717.56 rows=101 width=488) (actual time=104.950..105.327 rows=100 loops=1)
       Sort Key: t1.fivethous
       Sort Method: quicksort  Memory: 68kB
       ->  Hash Join  (cost=230.43..713.94 rows=101 width=488) (actual time=3.680..102.396 rows=100 loops=1)
             Hash Cond: (t2.unique2 = t1.unique2)
             ->  Seq Scan on tenk2 t2  (cost=0.00..445.00 rows=10000 width=244) (actual time=0.046..46.219 rows=10000 loops=1)
             ->  Hash  (cost=229.17..229.17 rows=101 width=244) (actual time=3.184..3.184 rows=100 loops=1)
                   Buckets: 1024  Batches: 1  Memory Usage: 27kB
                   ->  Bitmap Heap Scan on tenk1 t1  (cost=5.03..229.17 rows=101 width=244) (actual time=0.612..1.959 rows=100 loops=1)
                         Recheck Cond: (unique1 < 100)
                         ->  Bitmap Index Scan on tenk1_unique1  (cost=0.00..5.01 rows=101 width=0) (actual time=0.390..0.390 rows=100 loops=1)
                               Index Cond: (unique1 < 100)
     Total runtime: 107.392 ms
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  • 原文地址:https://www.cnblogs.com/gaojian/p/3134253.html
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