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当表数据量越来越大时查询速度会下降，像课本目录一样，在表的条件字段上创建索引，查询时能够快速定位感兴趣的数据所在的位置。索引的好处主要有加速带条件的查询，删除，更新，加速JOIN操作，加速外键约束更新和删除的操作等，但是索引也不是只有好处没有坏处，创建索引时会锁表，不仅将数据写入表中，还要创建索引，因此会在一定程度上影响写的性能。
Btree索引适合处理能够按顺序存储的数据的=,<,>,<=,>=,以及等效这些操作符的其他操作如BETWEEN，IN以及IS NULL和以字符串开头的模糊查询。Btree索引要想起作用where条件必须包含第一个索引列。
测试表：
test=# create table tbl_index(a bigint,b timestamp without time zone,c varchar(12)); CREATE TABLE test=# insert into tbl_index (a,b,c) select generate_series(1,3000000),clock_timestamp()::timestamp(0) without time zone,‘got u‘; INSERT 0 3000000
test=# \timing Timing is on.
示例1.创建索引前查询
test=# select * from tbl_index where a=3000000; a | b | c ---------+---------------------+------- 3000000 | 2016-06-29 14:54:00 | got u (1 row) Time: 303.729 ms
创建索引：
test=# create index idx_tbl_index_a on tbl_index using btree(a); CREATE INDEX Time: 3743.555 ms
示例2.创建索引后查询
test=# select * from tbl_index where a=3000000; a | b | c ---------+---------------------+------- 3000000 | 2016-06-29 14:54:00 | got u (1 row) Time: 2.316 ms
删除索引
test=# drop index idx_tbl_index_a ; DROP INDEX
创建组合索引
test=# create index idx_tbl_index_a_b on tbl_index using btree(a,b); CREATE INDEX Time: 2987.971 ms
使用explain命令输出查询计划，并使用analyze参数实际执行SQL语句。
示例3.按字段a查询
test=# explain analyze select * from tbl_index where a=3000000; QUERY PLAN ------------------------------------------------------------------------------------------------------------------------------ Index Scan using idx_tbl_index_a_b on tbl_index (cost=0.43..8.45 rows=1 width=22) (actual time=0.026..0.027 rows=1 loops=1) Index Cond: (a = 3000000) Planning time: 0.127 ms Execution time: 0.073 ms (4 rows) Time: 1.435 ms
示例4.按字段b查询
test=# explain analyze select * from tbl_index where b=‘2016-06-29 14:54:00‘; QUERY PLAN ----------------------------------------------------------------------------------------------------------------------------------- Gather (cost=1000.00..38495.00 rows=171000 width=22) (actual time=306.211..514.992 rows=172824 loops=1) Workers Planned: 2 Workers Launched: 2 -> Parallel Seq Scan on tbl_index (cost=0.00..21107.50 rows=71250 width=22) (actual time=284.671..296.463 rows=57608 loops=3) Filter: (b = ‘2016-06-29 14:54:00‘::timestamp without time zone) Rows Removed by Filter: 942392 Planning time: 0.191 ms Execution time: 524.130 ms (8 rows) Time: 525.623 ms
示例5.组合字段查询 a and b
test=# explain analyze select * from tbl_index where a=3000000 and b=‘2016-06-29 14:54:00‘; QUERY PLAN ------------------------------------------------------------------------------------------------------------------------------ Index Scan using idx_tbl_index_a_b on tbl_index (cost=0.43..8.45 rows=1 width=22) (actual time=0.031..0.033 rows=1 loops=1) Index Cond: ((a = 3000000) AND (b = ‘2016-06-29 14:54:00‘::timestamp without time zone)) Planning time: 0.169 ms Execution time: 0.075 ms (4 rows) Time: 1.173 ms
示例6.组合字段查询 a or b
test=# explain analyze select * from tbl_index where a=3000000 or b=‘2016-06-29 14:54:00‘; QUERY PLAN ------------------------------------------------------------------------------------------------------------------------------------- ------------- Bitmap Heap Scan on tbl_index (cost=34034.37..41369.38 rows=171001 width=22) (actual time=130.579..145.787 rows=172824 loops=1) Recheck Cond: ((a = 3000000) OR (b = ‘2016-06-29 14:54:00‘::timestamp without time zone)) Heap Blocks: exact=276 -> BitmapOr (cost=34034.37..34034.37 rows=171001 width=0) (actual time=130.480..130.480 rows=0 loops=1) -> Bitmap Index Scan on idx_tbl_index_a_b (cost=0.00..4.44 rows=1 width=0) (actual time=0.026..0.026 rows=1 loops=1) Index Cond: (a = 3000000) -> Bitmap Index Scan on idx_tbl_index_a_b (cost=0.00..33944.43 rows=171000 width=0) (actual time=130.452..130.452 rows=172 824 loops=1) Index Cond: (b = ‘2016-06-29 14:54:00‘::timestamp without time zone) Planning time: 0.215 ms Execution time: 153.074 ms (10 rows) Time: 154.065 ms
表中的索引实际是btree(a,b),从以上示例中可以看出，只有where条件包含索引的第一个字段，查询才会进行索引扫描，否则将进行全表扫描。示例5和示例6比较可知，组合索引字段间使用and和or（测试例使用pg9.6,记得低版本pg组合索引使用or查询会进行全表扫描，这个不知道是不是记错了）虽然都是索引扫描，但是and组合要比or组合查询速度更快。
删除组合索引，然后分别在a和b字段上创建索引
test=# drop index idx_tbl_index_a_b ; DROP INDEX Time: 36.017 ms test=# create index idx_tbl_index_a on tbl_index using btree (a); CREATE INDEX Time: 2277.276 ms test=# create index idx_tbl_index_b on tbl_index using btree (b); CREATE INDEX Time: 2278.055 ms
分别使用a and b和a or b进行查询
test=# explain analyze select * from tbl_index where a=3000000 and b=‘2016-06-29 14:54:00‘; QUERY PLAN ---------------------------------------------------------------------------------------------------------------------------- Index Scan using idx_tbl_index_a on tbl_index (cost=0.43..8.45 rows=1 width=22) (actual time=0.099..0.100 rows=1 loops=1) Index Cond: (a = 3000000) Filter: (b = ‘2016-06-29 14:54:00‘::timestamp without time zone) Planning time: 0.779 ms Execution time: 0.137 ms (5 rows) Time: 2.154 ms test=# explain analyze select * from tbl_index where a=3000000 or b=‘2016-06-29 14:54:00‘; QUERY PLAN ------------------------------------------------------------------------------------------------------------------------------------- -------- Bitmap Heap Scan on tbl_index (cost=1840.87..9175.88 rows=171001 width=22) (actual time=29.026..48.537 rows=172824 loops=1) Recheck Cond: ((a = 3000000) OR (b = ‘2016-06-29 14:54:00‘::timestamp without time zone)) Heap Blocks: exact=276 -> BitmapOr (cost=1840.87..1840.87 rows=171001 width=0) (actual time=28.968..28.968 rows=0 loops=1) -> Bitmap Index Scan on idx_tbl_index_a (cost=0.00..4.44 rows=1 width=0) (actual time=0.022..0.022 rows=1 loops=1) Index Cond: (a = 3000000) -> Bitmap Index Scan on idx_tbl_index_b (cost=0.00..1750.93 rows=171000 width=0) (actual time=28.943..28.943 rows=172824 l oops=1) Index Cond: (b = ‘2016-06-29 14:54:00‘::timestamp without time zone) Planning time: 0.142 ms Execution time: 57.446 ms (10 rows) Time: 58.151 ms
结果显示分别在a和b字段上创建索引与在(a,b)组合字段上创建索引相比，and查询性能下降，但是or的性能可以提升。
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原文地址：http://www.cnblogs.com/alianbog/p/5621749.html
