PostgreSQL creates a table partition

The steps to create a table partition are as follows:
1. Create the main table
CREATE TABLE users ( uid int not null primary key, name varchar(20));
2. Create a partitioned table (you must inherit from the main table above)
CREATE TABLE users_0 ( check (uid > = 0 and uid < 100) ) INHERITS (users);
CREATE TABLE users_1 ( check (uid > = 100)) INHERITS (users);
3. Create an index on a partitioned table CREATE INDEX users_0_uidindex on users_0(uid);
CREATE INDEX users_1_uidindex on users_1(uid);
4. Create rule RULE
CREATE RULE users_insert_0 AS
ON INSERT TO users WHERE
(uid > = 0 and uid < 100)
DO INSTEAD
INSERT INTO users_0 VALUES (NEW.uid,NEW.name);
CREATE RULE users_insert_1 AS
ON INSERT TO users WHERE
(uid > = 100)
DO INSTEAD
INSERT INTO users_1 VALUES (NEW.uid,NEW.name);
Now you can test the write data:
postgres=# INSERT INTO users VALUES (100,'smallfish');
INSERT 0 0
postgres=# INSERT INTO users VALUES (20,'aaaaa');
INSERT 0 0
postgres=# select * from users;
uid | name
-----+-----------
20 | aaaaa
100 | smallfish
(2 data columns)
postgres=# select * from users_0;
uid | name
-----+-------
20 | aaaaa
(1 data column)
postgres=# select * from users_1;
uid | name
-----+-----------
100 | smallfish
(1 data column)
Here the table partition can be calculated, but there is still a place to change, first look at the count query.
postgres=# EXPLAIN SELECT count(*) FROM users where uid < 100;
QUERY PLAN
---------------------------------------------------------------------------------------------
Aggregate (cost=62.75..62.76 rows=1 width=0)
- > Append (cost=6.52..60.55 rows=879 width=0)
- > Bitmap Heap Scan on users (cost=6.52..20.18 rows=293 width=0)
Recheck Cond: (uid < 100)
- > Bitmap Index Scan on users_pkey (cost=0.00..6.45 rows=293 width=0)
Index Cond: (uid < 100)
- > Bitmap Heap Scan on users_0 users (cost=6.52..20.18 rows=293 width=0)
Recheck Cond: (uid < 100)
- > Bitmap Index Scan on users_0_uidindex (cost=0.00..6.45 rows=293 width=0)
Index Cond: (uid < 100)
- > Bitmap Heap Scan on users_1 users (cost=6.52..20.18 rows=293 width=0)
Recheck Cond: (uid < 100)
- > Bitmap Index Scan on users_1_uidindex (cost=0.00..6.45 rows=293 width=0)
Index Cond: (uid < 100)
(14 data columns)
According to the original idea, uid is less than 100, so in theory, it should only query users_0 table, through which EXPLAIN can see other tables that he has scanned all partitions.
postgres=# SET constraint_exclusion = on;
SET
postgres=# EXPLAIN SELECT count(*) FROM users where uid < 100;
QUERY PLAN
---------------------------------------------------------------------------------------------
Aggregate (cost=41.83..41.84 rows=1 width=0)
- > Append (cost=6.52..40.37 rows=586 width=0)
- > Bitmap Heap Scan on users (cost=6.52..20.18 rows=293 width=0)
Recheck Cond: (uid < 100)
- > Bitmap Index Scan on users_pkey (cost=0.00..6.45 rows=293 width=0)
Index Cond: (uid < 100)
- > Bitmap Heap Scan on users_0 users (cost=6.52..20.18 rows=293 width=0)
Recheck Cond: (uid < 100)
- > Bitmap Index Scan on users_0_uidindex (cost=0.00..6.45 rows=293 width=0)
Index Cond: (uid < 100)
(10 data columns)
The whole process here is OK!