postgresql部分索引(Partial index)

主要是在满足条件的部分上建立索引,特别情况下的索引效率会很高.

创建partial index

db01=# create table t_index(id int,name varchar(30));
CREATE TABLE
db01=# insert into t_index select generate_series(1,1000),'name';
INSERT 0 1000
db01=# insert into t_index select generate_series(1001,1010),'name1';
INSERT 0 10
db01=# create index idx_t_index01 on t_index(name) where name != 'name';
CREATE INDEX
db01=# explain select * from t_index where name != 'name';
                                  QUERY PLAN                                   
-------------------------------------------------------------------------------
 Index Scan using idx_t_index01 on t_index  (cost=0.14..12.31 rows=10 width=9)
(1 row)

db01=# explain select * from t_index where name = 'name2';
                                 QUERY PLAN                                  
-----------------------------------------------------------------------------
 Index Scan using idx_t_index01 on t_index  (cost=0.14..4.15 rows=1 width=9)
   Index Cond: ((name)::text = 'name2'::text)
(2 rows)

db01=# explain select * from t_index where name = 'name';
                        QUERY PLAN                         
-----------------------------------------------------------
 Seq Scan on t_index  (cost=0.00..18.62 rows=1000 width=9)
   Filter: ((name)::text = 'name'::text)
(2 rows)

索引列跟where条件可以不同

db01=# drop index idx_t_index01;
DROP INDEX
db01=# create index idx_t_index02 on t_index(name) where id between 300 and 500;
CREATE INDEX
db01=# explain select * from t_index where name = 'name1' and id = 209;
                        QUERY PLAN                         
-----------------------------------------------------------
 Seq Scan on t_index  (cost=0.00..21.15 rows=1 width=9)
   Filter: (((name)::text = 'name1'::text) AND (id = 209))
(2 rows)

db01=# explain select * from t_index where name = 'name1' and id = 300;
                                 QUERY PLAN                                  
-----------------------------------------------------------------------------
 Index Scan using idx_t_index02 on t_index  (cost=0.14..8.19 rows=1 width=9)
   Index Cond: ((name)::text = 'name1'::text)
   Filter: (id = 300)
(3 rows)

从oracle 12C开始也支持这种索引.

索引的维护

db01=# update t_index set id = 3000 where id =300;
UPDATE 1
db01=# explain select * from t_index where id = 3000;
                       QUERY PLAN                       
--------------------------------------------------------
 Seq Scan on t_index  (cost=0.00..18.62 rows=1 width=9)
   Filter: (id = 3000)
(2 rows)

索引条目已经被移除索引,所以pg会自动维护相关索引

总结

部分索引的使用特别适用在大部分数据访问较少,只有部分数据需要经常访问的情况
比如未审核的单子,随着时间审核完成的越来越多,未审核的永远是那么些少数