postgresql 关于索引的简单测试

postgres=# create database c encoding 'sql_ascii' template template0 LC_collate='C' LC_ctype='C'; CREATE DATABASE postgres=# \c c You are now connected to database "c" as user "postgres". c=# create table t1(id int,name text); CREATE TABLE c=# insert into t1 select t::int,t::text from generate_series(1,1000000) as t; INSERT 0 1000000 c=# create index t1_name on t1(name); CREATE INDEX c=# vacuum ANALYZE t1; VACUUM c=# c=# c=# explain select * from t1 where name like '888%'; QUERY PLAN ------------------------------------------------------------------------ Index Scan using t1_name on t1 (cost=0.42..1540.99 rows=100 width=10) Index Cond: ((name >= '888'::text) AND (name < '889'::text)) Filter: (name ~~ '888%'::text) (3 rows) c=# explain select * from t1 where name like '%888'; QUERY PLAN --------------------------------------------------------- Seq Scan on t1 (cost=0.00..17905.00 rows=100 width=10) Filter: (name ~~ '%888'::text) (2 rows) c=# create database d encoding 'sql_ascii' template template0 LC_collate='zh_CN.UTF8' LC_ctype='zh_CN.UTF8'; CREATE DATABASE c=# c=# c=# \c d You are now connected to database "d" as user "postgres". d=# create table t1(id int,name text); CREATE TABLE d=# insert into t1 select t::int,1000000) as t; INSERT 0 1000000 d=# create index t1_name on t1(name); CREATE INDEX d=# vacuum ANALYZE t1 ; VACUUM d=# explain select * from t1 where name like '888%'; QUERY PLAN --------------------------------------------------------- Seq Scan on t1 (cost=0.00..17905.00 rows=100 width=10) Filter: (name ~~ '888%'::text) (2 rows) 用opclass建立索引，告诉数据库这列上要走索引，对于生产环境中有些情况很有效率 d=# drop index t1_name ;DROP INDEXd=# create index t1_name on t1 (name varchar_pattern_ops);CREATE INDEXd=# explain select * from t1 where name like '888%';QUERY PLAN --------------------------------------------------------------------------Bitmap Heap Scan on t1 (cost=22.80..2630.32 rows=100 width=10)Filter: (name ~~ '888%'::text)-> Bitmap Index Scan on t1_name (cost=0.00..22.78 rows=1035 width=0)Index Cond: ((name ~>=~ '888'::text) AND (name ~<~ '889'::text))(4 rows)d=# set enable_bitmapscan = off;SETd=# explain select * from t1 where name like '888%';QUERY PLAN ------------------------------------------------------------------------Index Scan using t1_name on t1 (cost=0.42..3815.71 rows=100 width=10)Index Cond: ((name ~>=~ '888'::text) AND (name ~<~ '889'::text))Filter: (name ~~ '888%'::text)(3 rows)