postgresql 索引状态_postgresql----索引失效

什么是索引失效？如果where过滤条件设置不合理，即使索引存在，且where过滤条件中包含索引列，也会导致全表扫描，索引不起作用。什么条件下会导致索引失效呢？

1.任何计算、函数、类型转换

2.!=

3.NOT，相当于使用函数

4.模糊查询通配符在开头

5.索引字段在表中占比较高

6.多字段btree索引查询条件不包含第一列

7.多字段索引查询条件使用OR(有时也会走索引扫描，但查询效率不高)

测试表

test=# \timing

Timingis on.

test=# create table tbl_index(a bigint,b timestamp without time zone ,c varchar(12));CREATE TABLETime:147.366ms

test=# insert into tbl_index select generate_series(1,10000000),clock_timestamp()::timestamp without time zone,'bit me';INSERT 0 10000000Time:30982.723 ms

1.任何计算、函数、类型转换

crtest=# create index idx_tbl_index_a ontbl_index (a);CREATE INDEXTime:19634.874ms

test=#

test=# explain analyze select * from tbl_index where a = 1;

QUERYPLAN

------------------------------------------------------------------------------------------------------------------------------

Index Scan using idx_tbl_index_a on tbl_index (cost=0.43..8.45 rows=1 width=23) (actual time=59.844..59.850 rows=1 loops=1)Index Cond: (a = 1)

Planning time:22.788ms

Execution time:60.011ms

(4rows)

Time:84.865ms

test=# explain analyze select * from tbl_index where a + 1 = 1;

QUERYPLAN

---------------------------------------------------------------------------------------------------------------------------------

Gather (cost=1000.00..84399.00 rows=50000 width=23) (actual time=7678.109..7678.109 rows=0 loops=1)

Workers Planned:2Workers Launched:2

-> Parallel Seq Scan on tbl_index (cost=0.00..78607.33 rows=20833 width=23) (actual time=7350.047..7350.047 rows=0 loops=3)

Filter: ((a+ 1) = 1)

Rows Removedby Filter: 3333333Planning time:0.112ms

Execution time:7688.615ms

(8rows)

Time:7780.024ms

test=# explain analyze select * from tbl_index where power(a,2) = 1;

QUERYPLAN

---------------------------------------------------------------------------------------------------------------------------------

Gather (cost=1000.00..94815.67 rows=50000 width=23) (actual time=47.516..6902.399 rows=1 loops=1)

Workers Planned:2Workers Launched:2

-> Parallel Seq Scan on tbl_index (cost=0.00..89024.00 rows=20833 width=23) (actual time=4607.894..6892.174 rows=0 loops=3)

Filter: (power((a)::double precision, '2'::double precision) &