小编cli*_*ime的帖子

这不是我使用 postgres 的一天。在我使用 PosgreSQL 9.2.3 的服务器机器上，我将 work_mem 设置为 4MB 以避免Sort Method: external merge Disk: 2072kB但它没有帮助：

cwu=# vacuum analyze web_city;
VACUUM
cwu=# SHOW work_mem;
 work_mem 
----------
 4MB
(1 row)
cwu=# explain analyze select count(*) from web_city GROUP BY (left(name,5));
                                                          QUERY PLAN                                                          
------------------------------------------------------------------------------------------------------------------------------
 GroupAggregate  (cost=18304.35..20487.34 rows=95562 width=10) (actual time=1557.871..1809.029 rows=64459 loops=1)
   ->  Sort  (cost=18304.35..18633.84 rows=131796 width=10) (actual time=1557.856..1707.069 rows=131796 loops=1)
         Sort Key: ("left"((name)::text, 5))
         Sort Method: external merge  Disk: 2072kB
         ->  Seq Scan on web_city  (cost=0.00..4842.45 rows=131796 width=10) (actual time=1.050..174.907 …