postgresql 数据库 alter table alter column set default 的一些实践

os: centos 7.4
db: postgresql 10.11

创建表后，有时需要对表进行 set default 或者 drop default 设置。

版本

# cat /etc/centos-release
CentOS Linux release 7.4.1708 (Core) 
# 
# su - postgres
$
$ psql -c "select version();"
                                                 version                                                  
----------------------------------------------------------------------------------------------------------
 PostgreSQL 10.11 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-39), 64-bit
(1 row)

create table

$ psql
psql (10.11)
Type "help" for help.

postgres=# \timing on
Timing is on.
postgres=# 
postgres=# drop table if exists tmp_t4;
DROP TABLE
Time: 313.287 ms

postgres=# create table tmp_t4( 
id    int8,
name  varchar(100),
memo1 varchar(100),
memo2 varchar(100)
);
CREATE TABLE
Time: 360.530 ms

postgres=# insert into tmp_t4 
select id,
       md5(id::varchar),
       md5(id::varchar),
	   null 
  from generate_series(1,10000000) as id;

INSERT 0 10000000
Time: 58346.187 ms (00:58.346)

postgres=# select * from tmp_t4 limit 5;

 id |               name               |              memo1               | memo2 
----+----------------------------------+----------------------------------+-------
  1 | c4ca4238a0b923820dcc509a6f75849b | c4ca4238a0b923820dcc509a6f75849b | 
  2 | c81e728d9d4c2f636f067f89cc14862c | c81e728d9d4c2f636f067f89cc14862c | 
  3 | eccbc87e4b5ce2fe28308fd9f2a7baf3 | eccbc87e4b5ce2fe28308fd9f2a7baf3 | 
  4 | a87ff679a2f3e71d9181a67b7542122c | a87ff679a2f3e71d9181a67b7542122c | 
  5 | e4da3b7fbbce2345d7772b0674a318d5 | e4da3b7fbbce2345d7772b0674a318d5 | 
(5 rows)

Time: 0.796 ms

postgres=# \d+ tmp_t4
                                          Table "public.tmp_t4"
 Column |          Type          | Collation | Nullable | Default | Storage  | Stats target | Description 
--------+------------------------+-----------+----------+---------+----------+--------------+-------------
 id     | bigint                 |           |          |         | plain    |              | 
 name   | character varying(100) |           |          |         | extended |              | 
 memo1  | character varying(100) |           |          |         | extended |              | 
 memo2  | character varying(100) |           |          |         | extended |              | 

postgres=# select oid,relname,relfilenode,relkind,relfrozenxid 
from pg_class pc where pc.relname='tmp_t4';

  oid   | relname | relfilenode | relkind | relfrozenxid 
--------+---------+-------------+---------+--------------
 123886 | tmp_t4  |      123886 | r       |       406624
(1 row)

Time: 0.579 ms

alter column set default

对列中不含null进行 set default 设置

postgres=# alter table tmp_t4 alter column memo1 set default 'aaa';
ALTER TABLE
Time: 1129.620 ms (00:01.130)

postgres=# \d+ tmp_t4
                                                   Table "public.tmp_t4"
 Column |          Type          | Collation | Nullable |         Default          | Storage  | Stats target | Description 
--------+------------------------+-----------+----------+--------------------------+----------+--------------+-------------
 id     | bigint                 |           |          |                          | plain    |              | 
 name   | character varying(100) |           |          |                          | extended |              | 
 memo1  | character varying(100) |           |          | 'aaa'::character varying | extended |              | 
 memo2  | character varying(100) |           |          |                          | extended |              | 
 
postgres=# select oid,relname,relfilenode,relkind,relfrozenxid 
from pg_class pc where pc.relname='tmp_t4';

  oid   | relname | relfilenode | relkind | relfrozenxid 
--------+---------+-------------+---------+--------------
 123886 | tmp_t4  |      123886 | r       |       406624
(1 row)

Time: 0.443 ms

对列中含null进行 default 设置

postgres=# alter table tmp_t4 alter column memo2 set default 'aaa';
ALTER TABLE
Time: 128.971 ms

alter column drop default

postgres=# \d+ tmp_t4
                                                   Table "public.tmp_t4"
 Column |          Type          | Collation | Nullable |         Default          | Storage  | Stats target | Description 
--------+------------------------+-----------+----------+--------------------------+----------+--------------+-------------
 id     | bigint                 |           |          |                          | plain    |              | 
 name   | character varying(100) |           |          |                          | extended |              | 
 memo1  | character varying(100) |           |          | 'aaa'::character varying | extended |              | 
 memo2  | character varying(100) |           |          | 'aaa'::character varying | extended |              | 

postgres=# alter table tmp_t4 alter column memo1 drop default;
ALTER TABLE
Time: 36.544 ms

postgres=# alter table tmp_t4 alter column memo2 drop default;
ALTER TABLE
Time: 59.798 ms

add column set default

postgres=# alter table tmp_t4 add column memo3 varchar(100) default 'aaa';
ALTER TABLE
Time: 73482.788 ms (01:13.483)

postgres=# select * from tmp_t4 limit 5;
 id |               name               |              memo1               | memo2 | memo3 
----+----------------------------------+----------------------------------+-------+-------
  1 | c4ca4238a0b923820dcc509a6f75849b | c4ca4238a0b923820dcc509a6f75849b |       | aaa
  2 | c81e728d9d4c2f636f067f89cc14862c | c81e728d9d4c2f636f067f89cc14862c |       | aaa
  3 | eccbc87e4b5ce2fe28308fd9f2a7baf3 | eccbc87e4b5ce2fe28308fd9f2a7baf3 |       | aaa
  4 | a87ff679a2f3e71d9181a67b7542122c | a87ff679a2f3e71d9181a67b7542122c |       | aaa
  5 | e4da3b7fbbce2345d7772b0674a318d5 | e4da3b7fbbce2345d7772b0674a318d5 |       | aaa
(5 rows)

Time: 0.619 ms

postgres=# select oid,relname,relfilenode,relkind,relfrozenxid 
from pg_class pc where pc.relname='tmp_t4';

  oid   | relname | relfilenode | relkind | relfrozenxid 
--------+---------+-------------+---------+--------------
 123886 | tmp_t4  |      123894 | r       |       406631
(1 row)

Time: 0.834 ms

relfilenode 由之前 123886 变为 123894

add column set default 确实要慎重

参考:
http://postgres.cn/docs/10/sql-altertable.html