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通过复合（两列）主键连接两个表时，我在查询计划中得到错误的基数估计。例子：

CREATE TABLE t1 AS SELECT x, x*2 AS x2 FROM generate_series(0, 1000) AS x;
ALTER TABLE t1 ADD PRIMARY KEY(x, x2);
ANALYZE t1;

CREATE TABLE t2 AS SELECT x, x*2 AS x2 FROM generate_series(0, 1000) AS x;
ALTER TABLE t2 ADD FOREIGN KEY (x, x2) REFERENCES t1(x,x2);
ANALYZE t2;

EXPLAIN ANALYZE
SELECT *
FROM t1 JOIN t2 USING (x, x2)

 QUERY PLAN                                                                                                    
 ------------------------------------------------------------------------------------------------------------- 
 Hash Join  (cost=30.02..52.55 rows=1 width=8) (actual time=0.660..1.551 rows=1001 loops=1)                    
   Hash Cond: ((t1.x = t2.x) AND (t1.x2 = t2.x2))                                                              
   ->  Seq Scan on t1  (cost=0.00..15.01 rows=1001 width=8) (actual time=0.021..0.260 rows=1001 loops=1)       
   ->  Hash  (cost=15.01..15.01 rows=1001 width=8) (actual time=0.620..0.620 rows=1001 loops=1)                
         Buckets: 1024  Batches: 1  Memory Usage: 40kB                                                         
         ->  Seq Scan on t2  (cost=0.00..15.01 rows=1001 width=8) (actual time=0.019..0.230 rows=1001 loops=1) 
 Total runtime: 1.679 ms

该计划期望返回一行，但实际上返回了 1001 行。这在简单查询中不是问题，但是在进行复杂查询时会导致查询计划非常慢。如何帮助查询优化器做得更好？

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2 回答 2

1

使用其中一列完全依赖于另一列的复合主键是一种“有趣”的设计。

无论如何，PostgreSQL 目前假设每列的选择性是相互独立的，因此将它们相乘（不管它们是否在同一个索引中，即使它是主键索引），我不知道一个很好的解决方法。

您可以使用这种迂回说法更接近真正的选择性：

EXPLAIN ANALYZE
SELECT *
FROM t1 JOIN t2 on (t1.x=t2.x and t1.x2 between t2.x2 and t2.x2);
于 2013-09-26T16:17:43.623 回答
0

另一种创建真正正交关键元素的方法：

CREATE TABLE t1 AS SELECT x/100 AS x, x%100 AS x2 FROM generate_series(0, 10000) AS x;
ALTER TABLE t1 ADD PRIMARY KEY(x, x2);
ANALYZE t1;

CREATE TABLE t2 AS SELECT x/100 AS x, x%100 AS x2 FROM generate_series(0, 10000) AS x;
ALTER TABLE t2 ADD PRIMARY KEY (x, x2) ; -- added PK
ALTER TABLE t2 ADD FOREIGN KEY (x, x2) REFERENCES t1(x,x2);

ANALYZE t2;
于 2013-09-26T16:39:12.057 回答