2 
EXPLAIN ANALYZE 
SELECT count(*) 
FROM "businesses" 
WHERE (
    source = 'facebook' 
    OR EXISTS( 
        SELECT * 
        FROM provider_business_map pbm 
        WHERE 
            pbm.hotstepper_business_id=businesses.id 
            AND pbm.provider_name='facebook' 
    )
);
PLAN                                                                                 
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=233538965.74..233538965.75 rows=1 width=0) (actual time=116169.720..116169.721 rows=1 loops=1)
   ->  Seq Scan on businesses  (cost=0.00..233521096.48 rows=7147706 width=0) (actual time=11.284..116165.646 rows=3693 loops=1)
         Filter: (((source)::text = 'facebook'::text) OR (alternatives: SubPlan 1 or hashed SubPlan 2))
         SubPlan 1
           ->  Index Scan using idx_provider_hotstepper_business on provider_business_map pbm  (cost=0.00..16.29 rows=1 width=0) (never executed)
                 Index Cond: (((provider_name)::text = 'facebook'::text) AND (hotstepper_business_id = businesses.id))
         SubPlan 2
           ->  Index Scan using idx_provider_hotstepper_business on provider_business_map pbm  (cost=0.00..16.28 rows=1 width=4) (actual time=0.045..5.685 rows=3858 loops=1)
                 Index Cond: ((provider_name)::text = 'facebook'::text)
 Total runtime: 116169.820 ms
(10 rows)

此查询需要一分钟多的时间，它的计数结果约为 3000。瓶颈似乎是顺序扫描，但我不确定数据库上需要什么索引来优化它。还值得注意的是，我没有调整 postgres，所以如果有任何调整可以帮助它可能值得考虑。虽然我的数据库是 15GB，但我不打算很快尝试将所有这些都放入内存中，所以我不确定更改与 RAM 相关的值是否会有很大帮助。

4

3 回答 3

2

OR 因性能不佳而臭名昭著。尝试将其拆分为两个表上两个完全独立的查询的联合：

SELECT COUNT(*) FROM (
    SELECT id
    FROM businesses 
    WHERE source = 'facebook'
    UNION   -- union makes the ids unique in the result
    SELECT hotstepper_business_id
    FROM provider_business_map
    WHERE provider_name = 'facebook'
    AND hotstepper_business_id IS NOT NULL
) x

如果hotstepper_business_id不能为空，您可以删除该行

AND hotstepper_business_id IS NOT NULL

如果您想要整个业务行，您可以简单地将上述查询包装为IN (...)：

SELECT * FROM businesses
WHERE ID IN (
    -- above inner query
)

但是一个性能更好的查询是修改上面的查询使用连接：

SELECT *
FROM businesses 
WHERE source = 'facebook'
UNION
SELECT b.*
FROM provider_business_map m
JOIN businesses b
  ON b.id = m.hotstepper_business_id
WHERE provider_name = 'facebook'
于 2013-07-03T06:29:25.307 回答
1

我至少会尝试将依赖子查询重写为；

SELECT COUNT(DISTINCT b.*)
FROM businesses b
LEFT JOIN provider_business_map pbm
  ON b.id=pbm.hotstepper_business_id
WHERE b.source = 'facebook'
  OR pbm.provider_name = 'facebook';

除非我误读了某些内容，否则businesses.id存在索引，但请确保索引也存在provider_business_map.hotstepper_business_id，businesses.source并且provider_business_map.provider_name为了获得最佳性能。

于 2013-07-03T06:13:57.443 回答
1 
create index index_name on businesses(source);

由于在超过 700 万行中有 3,693 行匹配，因此它可能会使用索引。不要忘记

analyse businesses;
于 2013-07-03T06:44:33.460 回答