Question

我在Postgresql中有这个表：

CREATE TABLE my_table
(
    id bigint NOT NULL,
    value bigint,
    CONSTRAINT my_table_pkey PRIMARY KEY (id)
);

my_table中有大约50000行。

问题是，为什么查询：

SELECT * FROM my_table WHERE id = COALESCE(null, id) and value = ?

比这个慢：

SELECT * FROM my_table WHERE value = ?

除了在app-layer中优化查询字符串之外，还有其他解决方案吗？

编辑：实际上，问题是如何重写查询select * from my_table where id=coalesce(?, id) and value=?以使最坏情况下的性能不低于Postgresql 9.0中的select * from my_table where value=?

Answer 1

尝试重写表单的查询

SELECT *
  FROM my_table
 WHERE value = ?
   AND (? IS NULL OR id = ?)

来自我自己的快速测试

INSERT INTO my_table select generate_series(1,50000),1;
UPDATE my_table SET value = id%17;

CREATE INDEX val_idx ON my_table(value);

VACUUM ANALYZE my_table;

\set idval 17
\set pval   0

explain analyze 
SELECT *
  FROM my_table
 WHERE value = :pval
   AND (:idval IS NULL OR id = :idval);

Index Scan using my_table_pkey on my_table  (cost=0.00..8.29 rows=1 width=16) (actual time=0.034..0.035 rows=1 loops=1)
   Index Cond: (id = 17)
   Filter: (value = 0)
 Total runtime: 0.064 ms

\set idval null

explain analyze 
SELECT *
  FROM my_table
 WHERE value = :pval
   AND (:idval IS NULL OR id = :idval);

Bitmap Heap Scan on my_table  (cost=58.59..635.62 rows=2882 width=16) (actual time=0.373..1.594 rows=2941 loops=1)
   Recheck Cond: (value = 0)
   ->  Bitmap Index Scan on validx  (cost=0.00..57.87 rows=2882 width=0) (actual time=0.324..0.324 rows=2941 loops=1)
         Index Cond: (value = 0)
 Total runtime: 1.811 ms

Answer 2

从创建类似的表，填充它，更新统计数据，最后查看EXPLAIN ANALYZE的输出，我看到的唯一区别是第一个查询过滤器如下：

Filter: ((id = COALESCE(id)) AND (value = 3))

，第二个过滤器如下：

Filter: (value = 3)

当列“值”上有索引时，我看到了截然不同的性能和执行计划。在第一种情况下

Bitmap Heap Scan on my_table  (cost=19.52..552.60 rows=5 width=16) (actual time=19.311..20.679 rows=1000 loops=1)
  Recheck Cond: (value = 3)
  Filter: (id = COALESCE(id))
  ->  Bitmap Index Scan on t2  (cost=0.00..19.52 rows=968 width=0) (actual time=19.260..19.260 rows=1000 loops=1)
        Index Cond: (value = 3)
Total runtime: 22.138 ms

并在第二个

Bitmap Heap Scan on my_table  (cost=19.76..550.42 rows=968 width=16) (actual time=0.302..1.293 rows=1000 loops=1)
  Recheck Cond: (value = 3)
  ->  Bitmap Index Scan on t2  (cost=0.00..19.52 rows=968 width=0) (actual time=0.276..0.276 rows=1000 loops=1)
        Index Cond: (value = 3)
Total runtime: 2.174 ms

所以我说它的速度较慢，因为db引擎a）评估COALESCE（）表达式而不是优化它，b）评估它涉及额外的过滤条件。

Postgresql COALESCE性能问题

2 个答案: