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