Postgres查询优化（强制索引扫描） [英] Postgres query optimization (forcing an index scan)

问题描述

以下是我的查询。我试图让它使用索引扫描，但它只会seq扫描。

Below is my query. I am trying to get it to use an index scan, but it will only seq scan.

顺便说一句 metric_data 表有1.3亿行。 指标表有大约2000行。

By the way the metric_data table has 130 million rows. The metrics table has about 2000 rows.

metric_data 表格列：

  metric_id integer
, t timestamp
, d double precision
, PRIMARY KEY (metric_id, t)

如何让此查询使用我的PRIMARY KEY索引？

How can I get this query to use my PRIMARY KEY index?

SELECT
    S.metric,
    D.t,
    D.d
FROM metric_data D
INNER JOIN metrics S
    ON S.id = D.metric_id
WHERE S.NAME = ANY (ARRAY ['cpu', 'mem'])
  AND D.t BETWEEN '2012-02-05 00:00:00'::TIMESTAMP
              AND '2012-05-05 00:00:00'::TIMESTAMP;

EXPLAIN：

Hash Join  (cost=271.30..3866384.25 rows=294973 width=25)
  Hash Cond: (d.metric_id = s.id)
  ->  Seq Scan on metric_data d  (cost=0.00..3753150.28 rows=29336784 width=20)
        Filter: ((t >= '2012-02-05 00:00:00'::timestamp without time zone)
             AND (t <= '2012-05-05 00:00:00'::timestamp without time zone))
  ->  Hash  (cost=270.44..270.44 rows=68 width=13)
        ->  Seq Scan on metrics s  (cost=0.00..270.44 rows=68 width=13)
              Filter: ((sym)::text = ANY ('{cpu,mem}'::text[]))

推荐答案

出于测试目的，您可以强制使用索引禁用顺序扫描 - 仅在当前会话中最佳：

For testing purposes you can force the use of the index by "disabling" sequential scans - best in your current session only:

SET enable_seqscan = OFF;

手册中的详细信息。我引用了禁用，因为您实际上无法禁用顺序表扫描。但是现在任何其他可用选项都适用于Postgres。这将证明（metric_id，t） 上的多列索引可以使用 - 只是不如前导列上的索引有效。

Details in the manual here. I quoted "disabling", because you cannot actually disable sequential table scans. But any other available option is now preferable for Postgres. This will prove that the multicolumn index on (metric_id, t) can be used - just not as effective as an index on the leading column.

通过切换 PRIMARY KEY 中的列顺序（以及用于实现它的索引），可能会得到更好的结果它背后的窗帘）到（t，metric_id）。或者使用相反的列创建附加索引。

You probably get better results by switching the order of columns in your PRIMARY KEY (and the index used to implement it behind the curtains with it) to (t, metric_id). Or create an additional index with reversed columns like that.

• 复合索引是否也适用于第一个字段的查询？

• Is a composite index also good for queries on the first field?

您通常不必通过手动干预强制更好的查询计划。如果设置 enable_seqscan = OFF 会导致很多更好的计划，那么您的数据库中的某些内容可能就不对了。请考虑以下相关答案：

You do not normally have to force better query plans by manual intervention. If setting enable_seqscan = OFF leads to a much better plan, something is probably not right in your database. Consider this related answer:

• 保持PostgreSQL有时选择错误的查询计划

• Keep PostgreSQL from sometimes choosing a bad query plan

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