我在RHEL 5.8 OS上运行postgresql-9.1.6。我得到了一个声明执行seq扫描哪一列索引。为什么不使用我的索引
Table "public.table"
Column | Type | Modifiers
----------+-----------------------+-----------------------------------------
col1 | character(3) | not null
col2 | character varying(20) | not null
col3 | character varying(20) |
col4 | character(1) | default 0
Indexes:
"table_pkey" PRIMARY KEY, btree (col1, col2)
postgres=# explain analyze select * from table where col1=right('10000081',3);
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------
Seq Scan on table (cost=0.00..31053.24 rows=5650 width=286) (actual time=3.221..429.950 rows=110008 loops=1)
Filter: ((col1)::text = '081'::text)
Total runtime: 435.904 ms
(3 rows)
postgres=# explain analyze select * from table where col1=right('10000081',3)::char(3);
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on table (cost=3097.81..18602.98 rows=112173 width=286) (actual time=18.125..32.707 rows=110008 loops=1)
Recheck Cond: (col1 = '081'::character(3))
-> Bitmap Index Scan on table_pkey (cost=0.00..3069.77 rows=112173 width=0) (actual time=17.846..17.846 rows=110008 loops=1)
Index Cond: (col1 = '081'::character(3))
Total runtime: 38.640 ms
(5 rows)
我发现[改变柱]是溶液中的一个....
postgres=# alter table table alter column col1 type varchar(3);
ALTER TABLE
postgres=# explain analyze select * from table where col1=right('10000081',3);
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on table (cost=160.26..10902.32 rows=5650 width=295) (actual time=20.249..41.658 rows=110008 loops=1)
Recheck Cond: ((col1)::text = '081'::text)
-> Bitmap Index Scan on table_pkey (cost=0.00..158.85 rows=5650 width=0) (actual time=20.007..20.007 rows=110008 loops=1)
Index Cond: ((col1)::text = '081'::text)
Total runtime: 47.408 ms
(5 rows)
不知什么是WHY ???
请向我们展示您希望查询使用的索引的定义。 –
对不起,表格描述被遗漏了... – KIM