sql - 为什么创建不相关的索引会使我的查询更快？

Question

我有一张 table :

debts (
    name       text,
    inv_no     integer,
    inv_type   text,
    status     text,
);

我有以下选择:

SELECT COUNT(*) FROM debts WHERE name = '...' AND inv_no = 100 AND inv_type = '...';

为了优化其他东西，我添加了:

CREATE INDEX ON debt (status);

我在 SELECT 的任何地方都没有提到状态，但正在运行......

EXPLAIN SELECT COUNT(*)... (as above)

...在创建索引之前和之后，成本从 16.65..16.66 变为 1.25..1.26。为什么？

完整的解释(分析，详细)之前/之后:

之前:

QUERY PLAN
----------
 Aggregate  (cost=16.65..16.66 rows=1 width=0) (actual time=0.126..0.128 rows=1 loops=1)
   Output: count(*)
   ->  Seq Scan on ab123456.debts  (cost=0.00..16.65 rows=1 width=0) (actual time=0.106..0.106 rows=0 loops=1)
         Output: name, inv_no, inv_type, status
         Filter: ((debts.name = '...'::text) AND (debts.inv_type = '...'::text) AND (debts.inv_no = 100))
 Total runtime: 0.387 ms

之后:

QUERY PLAN
----------
 Aggregate  (cost=1.25..1.26 rows=1 width=0) (actual time=0.031..0.033 rows=1 loops=1)
   Output: count(*)
   ->  Seq Scan on ab123456.debts  (cost=0.00..1.25 rows=1 width=0) (actual time=0.024..0.024 rows=0 loops=1)
         Output: name, inv_no, inv_type, status
         Filter: ((debts.name = '...'::text) AND (debts.inv_type = '...'::text) AND (debts.inv_no = 100))
 Total runtime: 0.118 ms

Answer 1

一些实用程序语句(包括 CREATE INDEX!)在执行时更新表统计信息。 The manual:

For efficiency reasons, reltuples and relpages are not updated on-the-fly, and so they usually contain somewhat out-of-date values. They are updated by VACUUM, ANALYZE, and a few DDL commands such as CREATE INDEX.

大胆强调我的。因此，即使您的索引看起来完全不相关，更新的表统计信息也会产生影响 - 特别是对 count() 而言，它主要取决于所提到的两个统计信息。

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