?PostgreSQL对空表的行估算

PostgreSQL对空表的行估算逻辑挺奇怪的，固定认为有2000多行。

postgres=# create table tbchj(id int);
CREATE TABLE
postgres=# explain select * from tbchj;
                       QUERY PLAN
---------------------------------------------------------
 Seq Scan on tbchj (cost=0.00..35.50 rows=2550 width=4)
(1 row)

postgres=# explain select * from tbchj where id =9;
                      QUERY PLAN
-------------------------------------------------------
 Seq Scan on tbchj (cost=0.00..41.88 rows=13 width=4)
   Filter: (id = 9)
(2 rows)

postgres=# analyze tbchj;
ANALYZE
postgres=# explain select * from tbchj;
                       QUERY PLAN
---------------------------------------------------------
 Seq Scan on tbchj (cost=0.00..35.50 rows=2550 width=4)
(1 row)

插入数据后，行估算才能调整正确

postgres=# insert into tbchj values(1);
INSERT 0 1
postgres=# explain select * from tbchj;
                       QUERY PLAN
---------------------------------------------------------
 Seq Scan on tbchj (cost=0.00..35.50 rows=2550 width=4)
(1 row)

postgres=# analyze tbchj;
ANALYZE
postgres=# explain select * from tbchj;
                     QUERY PLAN
-----------------------------------------------------
 Seq Scan on tbchj (cost=0.00..1.01 rows=1 width=4)
(1 row)