PostgreSQL禁用全表扫描"/>
PostgreSQL禁用全表扫描
作者:瀚高PG实验室 (Highgo PG Lab)- 徐云鹤
PostgreSQL可以通过一些设置来禁用全表扫描(FULL SCAN/Seq Scan)
注意:
设置此功能后不是完全避免全表扫描,而是只要有不通过全表扫描能得出结果的就不走全表扫描。如果什么路都不通,那肯定得全表扫描,不然怎么获取数据。
而且并不是不走全表扫描性能就一定好。
下面展示下这个功能:
查询表结构:
highgo=# \d testTable testColumn | Type | Modifiers
-------------+--------------------------------+-----------G | character varying(50) | A | character varying(12) | M | timestamp(0) without time zone | W | character varying(5) |
Indexes:"s__x0" btree ("G", "A", "M", "W")
先检查视图:
highgo=# select * from pg_db_role_setting ;setdatabase | setrole | setconfig
-------------+---------+-----------
(0 rows)
查询执行计划:
highgo=# explain select "G","Z" from test where "G"='PG';QUERY PLAN
------------------------------------------------------------------------------Seq Scan on test (cost=0.00..3.11 rows=1 width=72)Filter: (("G")::text = '7e'::text)
(2 rows)
Seq Scan on test (cost=0.00..3.11 rows=1 width=72)Filter: (("G")::text = '7e'::text)
(2 rows)
对用户进行限制:
highgo=# alter role xyh set enable_seqscan =off;
ALTER ROLEhighgo=# select * from pg_db_role_setting ;setdatabase | setrole | setconfig
-------------+---------+----------------------0 | 26171 | {enable_seqscan=off}
enable_seqscan =off;
ALTER ROLEhighgo=# select * from pg_db_role_setting ;setdatabase | setrole | setconfig
-------------+---------+----------------------0 | 26171 | {enable_seqscan=off}
再次查询执行计划:
highgo=# explain select "G","Z" from test where "G"='7e';QUERY PLAN
------------------------------------------------------------------------------Index Scan using "s__x0" on test (cost=0.14..8.15 rows=1 width=72)Index Cond: (("G")::text = '7e'::text)
(2 rows)
Index Scan using "s__x0" on test (cost=0.14..8.15 rows=1 width=72)Index Cond: (("G")::text = '7e'::text)
(2 rows)
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PostgreSQL禁用全表扫描
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