當表數據量越來越大時查詢速度會下降，像課本目錄一樣，在表的條件字段上創建索引，查詢時能夠快速定位感興趣的數據所在的位置。索引的好處主要有加速帶條件的查詢，刪除，更新，加速JOIN操作，加速外鍵約束更新和刪除的操作等，但是索引也不是只有好處沒有壞處，創建索引時會鎖表，不僅將數據寫入表中，還要創建索引，因此會在一定程度上影響寫的性能。
Btree索引適合處理能夠按順序存儲的數據的=,<,>,<=,>=,以及等效這些操作符的其他操作如BETWEEN，IN以及IS NULL和以字符串開頭的模糊查詢。Btree索引要想起作用where條件必須包含第一個索引列。
測試表：
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.創建索引前查詢
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
創建索引：
test=# create index idx_tbl_index_a on tbl_index using btree(a); CREATE INDEX Time: 3743.555 ms
示例2.創建索引后查詢
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
刪除索引
test=# drop index idx_tbl_index_a ; DROP INDEX
創建組合索引
test=# create index idx_tbl_index_a_b on tbl_index using btree(a,b); CREATE INDEX Time: 2987.971 ms
使用explain命令輸出查詢計划，並使用analyze參數實際執行SQL語句。
示例3.按字段a查詢
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查詢
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.組合字段查詢 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.組合字段查詢 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
表中的索引實際是btree(a,b),從以上示例中可以看出，只有where條件包含索引的第一個字段，查詢才會進行索引掃描，否則將進行全表掃描。示例5和示例6比較可知，組合索引字段間使用and和or（測試例使用pg9.6,記得低版本pg組合索引使用or查詢會進行全表掃描，這個不知道是不是記錯了）雖然都是索引掃描，但是and組合要比or組合查詢速度更快。
刪除組合索引，然后分別在a和b字段上創建索引
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
分別使用a and b和a or b進行查詢
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
結果顯示分別在a和b字段上創建索引與在(a,b)組合字段上創建索引相比，and查詢性能下降，但是or的性能可以提升。
PS：主鍵和唯一鍵會自動創建Btree索引，無需另外單獨再為主鍵和唯一鍵創建索引。