performance - postgresql COUNT(DISTINCT ...) 非常慢

Question

我有一个非常简单的 SQL 查询：

SELECT COUNT(DISTINCT x) FROM table;

我的表有大约 150 万行。这个查询运行得很慢；大约需要 7.5 秒，相比之下

 SELECT COUNT(x) FROM table;

这大约需要 435 毫秒。有什么方法可以更改我的查询以提高性能？我尝试分组并定期计数，以及在 x 上放置索引；两者都有相同的 7.5 秒执行时间。

Answer 1

你可以使用这个：

SELECT COUNT(*) FROM (SELECT DISTINCT column_name FROM table_name) AS temp;

这比：

COUNT(DISTINCT column_name)

Answer 2

-- My default settings (this is basically a single-session machine, so work_mem is pretty high)
SET effective_cache_size='2048MB';
SET work_mem='16MB';

\echo original
EXPLAIN ANALYZE
SELECT
        COUNT (distinct val) as aantal
FROM one
        ;

\echo group by+count(*)
EXPLAIN ANALYZE
SELECT
        distinct val
       -- , COUNT(*)
FROM one
GROUP BY val;

\echo with CTE
EXPLAIN ANALYZE
WITH agg AS (
    SELECT distinct val
    FROM one
    GROUP BY val
    )
SELECT COUNT (*) as aantal
FROM agg
        ;

结果：

original                                                      QUERY PLAN                                                      
----------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=36448.06..36448.07 rows=1 width=4) (actual time=1766.472..1766.472 rows=1 loops=1)
   ->  Seq Scan on one  (cost=0.00..32698.45 rows=1499845 width=4) (actual time=31.371..185.914 rows=1499845 loops=1)
 Total runtime: 1766.642 ms
(3 rows)

group by+count(*)
                                                         QUERY PLAN                                                         
----------------------------------------------------------------------------------------------------------------------------
 HashAggregate  (cost=36464.31..36477.31 rows=1300 width=4) (actual time=412.470..412.598 rows=1300 loops=1)
   ->  HashAggregate  (cost=36448.06..36461.06 rows=1300 width=4) (actual time=412.066..412.203 rows=1300 loops=1)
         ->  Seq Scan on one  (cost=0.00..32698.45 rows=1499845 width=4) (actual time=26.134..166.846 rows=1499845 loops=1)
 Total runtime: 412.686 ms
(4 rows)

with CTE
                                                             QUERY PLAN                                                             
------------------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=36506.56..36506.57 rows=1 width=0) (actual time=408.239..408.239 rows=1 loops=1)
   CTE agg
     ->  HashAggregate  (cost=36464.31..36477.31 rows=1300 width=4) (actual time=407.704..407.847 rows=1300 loops=1)
           ->  HashAggregate  (cost=36448.06..36461.06 rows=1300 width=4) (actual time=407.320..407.467 rows=1300 loops=1)
                 ->  Seq Scan on one  (cost=0.00..32698.45 rows=1499845 width=4) (actual time=24.321..165.256 rows=1499845 loops=1)
       ->  CTE Scan on agg  (cost=0.00..26.00 rows=1300 width=0) (actual time=407.707..408.154 rows=1300 loops=1)
     Total runtime: 408.300 ms
    (7 rows)

与 CTE 相同的计划可能也可以通过其他方法（窗口函数）生成

Answer 3

如果您count(distinct(x))的速度明显慢于count(x)那么您可以通过在不同的表中维护 x 值计数来加速此查询，例如table_name_x_counts (x integer not null, x_count int not null)，使用触发器。但是您的写入性能会受到影响，如果您x在单个事务中更新多个值，那么您需要以某种明确的顺序执行此操作以避免可能的死锁。

Answer 4

我也在寻找相同的答案，因为在某些时候我需要具有不同值的 total_count 以及 limit/offset。

因为它有点棘手 - 获得具有不同值的总计数以及限制/偏移量。通常很难通过限制/偏移来获得总计数。终于我有办法了-

SELECT DISTINCT COUNT(*) OVER() as total_count, * FROM table_name limit 2 offset 0;

查询性能也很高。