4

我多次听说 postgres 处理存在查询的速度比left join还要快。 http://archives.postgresql.org/pgsql-performance/2002-12/msg00185.php

对于一张表聚合来说，这绝对是正确的。

但在我们的例子中，它们不仅仅是一个相同的查询构建，它使postgres永远挂起：

explain 
SELECT count(DISTINCT "groups".id) AS count_all 
FROM "groups"
WHERE (exists(
    select * from products p where groups.id = p.group_id AND exists(
        select * from products_categories pc where p.id = pc.product_id AND pc.category_id in (2,3))) AND groups.id != 3)

结果：

 Aggregate  (cost=26413436.66..26413436.67 rows=1 width=4)
   ->  Seq Scan on groups  (cost=0.00..26413403.84 rows=13126 width=4)
         Filter: ((id <> 3) AND (subplan))
         SubPlan
           ->  Index Scan using index_products_on_group_id on products p  (cost=0.00..1006.13 rows=1 width=1483)
                 Index Cond: ($1 = group_id)
                 Filter: (subplan)
                 SubPlan
                   ->  Seq Scan on products_categories pc  (cost=0.00..498.49 rows=1 width=8)
                         Filter: ((category_id = ANY ('{2,3}'::integer[])) AND ($0 = product_id))

这是执行时间长得令人难以置信的根本原因吗？这是某种配置问题吗？

谢谢，博格丹。

4

1 回答 1

3

好吧，对于“组”中的每一行，postgresql 都在对 products_categories 进行全面扫描，这并不好。不一定是配置问题，但也许可以在不嵌套子查询的情况下声明查询？

SELECT count(DISTINCT "groups".id) AS count_all 
FROM "groups"
WHERE exists(
    select 1 from products p where groups.id = p.group_id
             join products_categories pc on pc.product_id = p.id
    where pc.category_id in (2,3)
    ) and groups.id <> 3

也products_categories有索引product_id吗？

于 2010-06-24T14:42:59.443 回答