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