小编Den*_*dov的帖子

我创建了一个 36M 行的表，列上有一个索引type：

CREATE TABLE items AS
  SELECT
    (random()*36000000)::integer AS id,
    (random()*10000)::integer AS type,
    md5(random()::text) AS s
  FROM
    generate_series(1,36000000);
CREATE INDEX items_type_idx ON items USING btree ("type");

我运行这个简单的查询并期望 postgresql 使用我的索引：

explain select count(*) from "items" group by "type";

但是查询计划器决定使用 Seq Scan 代替：

HashAggregate  (cost=734592.00..734627.90 rows=3590 width=12) (actual time=6477.913..6478.344 rows=3601 loops=1)
  Group Key: type
  ->  Seq Scan on items  (cost=0.00..554593.00 rows=35999800 width=4) (actual time=0.044..1820.522 rows=36000000 loops=1)
Planning time: 0.107 ms
Execution time: 6478.525 ms

无解释时间： 5s 979ms

我从这里 …