为什么创建不相关的索引会使我的查询更快？

Question

我有一个 table:

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

我有一个关注 select:

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

为了优化其他内容，我添加了：

CREATE INDEX ON debt (status);

我在 SELECT 中没有提到状态，但是运行...

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

...在创建索引之前和之后，我将成本 16.65..16.66 更改为 1.25..1.26。为什么？

完整 explain (analyze, verbose) before/after:

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!）在执行时会更新 table 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For efficiency reasons, reltuples and relpages are not updated on-the-fly, and so they usually contain somewhat out-of-date values.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，更新后的 table 统计数据也会产生影响 - 特别是 count()，这主要取决于所提到的两个统计数据。

为什么创建不相关的索引会使我的查询更快？

Why does creating an unrelated index make my query faster?

sql

postgresql

indexing

optimization

sql-execution-plan