【PostgreSQL表增加/删除字段是否会重写表】

一、表添加字段

1.PostgreSQL10版本及以下

新增不带默认值的列
PostgreSQL 10 版本前表新增不带默认值的列不需要重写表，只需要更新数据字典，因此能瞬间执行。如果不带默认值，则会填充空值。

新增带默认值的列
如果新增的字段带默认值，则需要重写表。表越大，执行时间越长。重写表会对表加Access Exclusive锁，期间整张表是无法访问的。

如果是生产环境下给大表添加带 Default 值的字段可能影响比较大，通常先添加不带 Default值的字段，然后写函数批量刷新新增字段的默认值。也可以在业务量较低的时候或者申请割接窗口停业务一次性完成带DEFAUL值字段的新增。除此之外，如果有必要，可以清理一下可能堵塞DDL的长事务或者后台任务(例如autovacuum)。

1.PostgreSQL11版本及以上

PostgreSQL 11 版本这方面进一步增强，表新增带非空默认值的字段不再需要重写表，Release 中的说明如下：

Release中的说明
Allow ALTER TABLE to add a column with a non-null default without a table rewrite

主要的实现方法是：在系统表 pg_catalog.pg_attribute 中添加了两个字段：atthasmissing 和 attmissingval。新增列的默认值会记录到attmissingval，并且对应的atthasmissing会被设置为true。查询时如果tuple中不包含对应的列，则会返回attmissingval的值。

但是如果表之后因为其他的操作导致表被重写，例如vacuum full，则相应的atthasmissing和attmissingval属性将会被清除。

postgres=# create table t(id int, name varchar(20));
CREATE TABLE
postgres=# insert into t select generate_series(1,10),left(md5(random()::text),20);
INSERT 0 10
postgres=# select attname, attmissingval, atthasmissing FROM pg_attribute WHERE attnum > 0 and attrelid = 't'::regclass;
 attname | attmissingval | atthasmissing
---------+---------------+---------------
 id      |               | f
 name    |               | f
(2 rows)

postgres=# alter table t add column test_default int default 1;
ALTER TABLE
postgres=# alter table t add column test_nodefault int;
ALTER TABLE
postgres=# select attname, attmissingval, atthasmissing FROM pg_attribute WHERE attnum > 0 and attrelid = 't'::regclass;
    attname     | attmissingval | atthasmissing
----------------+---------------+---------------
 id             |               | f
 name           |               | f
 test_default   | {1}           | t
 test_nodefault |               | f
(4 rows)

二、表删除字段

1.删除字段分析

删除一个列时，并不会重建表（逐行扫表重写），而是将pg_attribute中对应的列的attname字段修改为… pg.dropped.idx… ，attisdropped标记为true，查询时会跳过该列。因此，删除列操作可以很快完成。

postgres=# \d t
                            Table "public.t"
     Column     |         Type          | Collation | Nullable | Default
----------------+-----------------------+-----------+----------+---------
 id             | integer               |           |          |
 name           | character varying(20) |           |          |
 test_default   | integer               |           |          | 1
 test_nodefault | integer               |           |          |

postgres=# select attname, attmissingval, atthasmissing FROM pg_attribute WHERE attnum > 0 and attrelid = 't'::regclass;
    attname     | attmissingval | atthasmissing
----------------+---------------+---------------
 id             |               | f
 name           |               | f
 test_default   | {1}           | t
 test_nodefault |               | f
(4 rows)

postgres=# alter table t drop column test_nodefault;
ALTER TABLE
postgres=# select attname, attmissingval, atthasmissing FROM pg_attribute WHERE attnum > 0 and attrelid = 't'::regclass;
           attname            | attmissingval | atthasmissing
------------------------------+---------------+---------------
 id                           |               | f
 name                         |               | f
 test_default                 | {1}           | t
 ........pg.dropped.4........ |               | f
(4 rows)

但是drop后的列，pg_attribute里的……… pg.dropped.idx… 记录不会因为做了vacuum full而被移除，这一点和11版本增加带默认值的列不同。

postgres=# select attname, attmissingval, atthasmissing FROM pg_attribute WHERE attnum > 0 and attrelid = 't'::regclass;
           attname            | attmissingval | atthasmissing
------------------------------+---------------+---------------
 id                           |               | f
 name                         |               | f
 test_default                 | {1}           | t
 ........pg.dropped.4........ |               | f
(4 rows)

postgres=# vacuum full t;
VACUUM
postgres=# select attname, attmissingval, atthasmissing FROM pg_attribute WHERE attnum > 0 and attrelid = 't'::regclass;
           attname            | attmissingval | atthasmissing
------------------------------+---------------+---------------
 id                           |               | f
 name                         |               | f
 test_default                 |               | f
 ........pg.dropped.4........ |               | f
(4 rows)

2.vacuum full对删除字段的影响

不过做过了vacuum full 操作已经把这列对应的数据清理掉了，即使通过更新系统表让列恢复，数据也不能恢复了。（vacuum不会导致列数据恢复不了）

如下是没做vacuum full和做过了vacuum full的列恢复情况。可以看到未做vacuum full的列可以正常恢复，但是做过了vacuum full的列虽然可以恢复，但是列原来存储的数据已经不存在了，用NULL填充。

（1）删除字段后未做vacuum full

//创建测试表

postgres=# create table t(id int, name varchar(20));
CREATE TABLE
postgres=# insert into t select generate_series(1,10),left(md5(random()::text),20);
INSERT 0 10
postgres=# select * from t;
 id |         name
----+----------------------
  1 | bcbd19340969fda7c9c4
  2 | a9f514a971eae3937def
  3 | 20d53f04cce29b1e2984
  4 | 912dd222955487de27a7
  5 | 5b18d6e2c9b22d34884f
  6 | 4c7db5a43de739511864
  7 | b2f1b83e98bdfdce8bce
  8 | 1fe69e2bec216de50f29
  9 | 689ecc14d87ae81fc0d1
 10 | 0ba176b240d0875da3e8
(10 rows)

查询列信息，attrelid和pg_class.oid关联，即表的oid。atttypid和pg_type.oid关联，列类型的oid，1043即对应varchar类型。attisdropped为t则表示该列被删除且不再有效。一个删除的列仍然物理存在于表中，但是会被分析器忽略并因此无法通过SQL访问。


postgres=# select attrelid,attname,atttypid,attnum,attisdropped FROM pg_attribute WHERE attnum > 0 and attrelid = 't'::regclass;
 attrelid | attname | atttypid | attnum | attisdropped
----------+---------+----------+--------+--------------
    16395 | id      |       23 |      1 | f
    16395 | name    |     1043 |      2 | f
(2 rows)

postgres=# select * from pg_type where oid=1043;
 oid  | typname | typnamespace | typowner | typlen | typbyval | typtype | typcategory | typispreferred | typisdefined | typdelim | typrelid | typsubscript |
typelem | typarray | typinput  | typoutput  | typreceive  |   typsend   |    typmodin     |    typmodout     | typanalyze | typalign | typstorage | typnotnul
l | typbasetype | typtypmod | typndims | typcollation | typdefaultbin | typdefault | typacl
------+---------+--------------+----------+--------+----------+---------+-------------+----------------+--------------+----------+----------+--------------+-
--------+----------+-----------+------------+-------------+-------------+-----------------+------------------+------------+----------+------------+----------
--+-------------+-----------+----------+--------------+---------------+------------+--------
 1043 | varchar |           11 |       10 |     -1 | f        | b       | S           | f              | t            | ,        |        0 | -            |
      0 |     1015 | varcharin | varcharout | varcharrecv | varcharsend | varchartypmodin | varchartypmodout | -          | i        | x          | f
  |           0 |        -1 |        0 |          100 |               |            |
(1 row)


//删除列

postgres=# alter table t drop column name;
ALTER TABLE
postgres=# select * from t;
 id
----
  1
  2
  3
  4
  5
  6
  7
  8
  9
 10
(10 rows)

postgres=# select attrelid,attname,atttypid,attnum,attisdropped FROM pg_attribute WHERE attnum > 0 and attrelid = 't'::regclass;
 attrelid |           attname            | atttypid | attnum | attisdropped
----------+------------------------------+----------+--------+--------------
    16395 | id                           |       23 |      1 | f
    16395 | ........pg.dropped.2........ |        0 |      2 | t
(2 rows)


//更新系统表，恢复删除的列

postgres=# update pg_attribute set attname='name',atttypid=1043,attisdropped='f' where attrelid='t'::regclass and attnum=2;
UPDATE 1
postgres=# select attrelid,attname,atttypid,attnum,attisdropped FROM pg_attribute WHERE attnum > 0 and attrelid = 't'::regclass;
 attrelid | attname | atttypid | attnum | attisdropped
----------+---------+----------+--------+--------------
    16395 | id      |       23 |      1 | f
    16395 | name    |     1043 |      2 | f
(2 rows)

postgres=# select * from t;
 id |         name
----+----------------------
  1 | bcbd19340969fda7c9c4
  2 | a9f514a971eae3937def
  3 | 20d53f04cce29b1e2984
  4 | 912dd222955487de27a7
  5 | 5b18d6e2c9b22d34884f
  6 | 4c7db5a43de739511864
  7 | b2f1b83e98bdfdce8bce
  8 | 1fe69e2bec216de50f29
  9 | 689ecc14d87ae81fc0d1
 10 | 0ba176b240d0875da3e8
(10 rows)

（2）删除字段后做过vacuum full

//删除列
postgres=# alter table t drop column name;
ALTER TABLE
postgres=# select attrelid,attname,atttypid,attnum,attisdropped FROM pg_attribute WHERE attnum > 0 and attrelid = 't'::regclass;
 attrelid |           attname            | atttypid | attnum | attisdropped
----------+------------------------------+----------+--------+--------------
    16395 | id                           |       23 |      1 | f
    16395 | ........pg.dropped.2........ |        0 |      2 | t
(2 rows)

可以看到做过了vacuum full后，对应的pg_attribute里的这条已经删除的列的信息依旧没有清理掉。

postgres=# vacuum full t;
VACUUM
postgres=# select attrelid,attname,atttypid,attnum,attisdropped FROM pg_attribute WHERE attnum > 0 and attrelid = 't'::regclass;
 attrelid |           attname            | atttypid | attnum | attisdropped
----------+------------------------------+----------+--------+--------------
    16395 | id                           |       23 |      1 | f
    16395 | ........pg.dropped.2........ |        0 |      2 | t
(2 rows)

//恢复数据
postgres=# update pg_attribute set attname='name',atttypid=1043,attisdropped='f' where attrelid='t'::regclass and attnum=2;
UPDATE 1
postgres=# select * from pg_type where oid=1043;
 oid  | typname | typnamespace | typowner | typlen | typbyval | typtype | typcategory | typispreferred | typisdefined | typdelim | typrelid | typsubscript |
typelem | typarray | typinput  | typoutput  | typreceive  |   typsend   |    typmodin     |    typmodout     | typanalyze | typalign | typstorage | typnotnul
l | typbasetype | typtypmod | typndims | typcollation | typdefaultbin | typdefault | typacl
------+---------+--------------+----------+--------+----------+---------+-------------+----------------+--------------+----------+----------+--------------+-
--------+----------+-----------+------------+-------------+-------------+-----------------+------------------+------------+----------+------------+----------
--+-------------+-----------+----------+--------------+---------------+------------+--------
 1043 | varchar |           11 |       10 |     -1 | f        | b       | S           | f              | t            | ,        |        0 | -            |
      0 |     1015 | varcharin | varcharout | varcharrecv | varcharsend | varchartypmodin | varchartypmodout | -          | i        | x          | f
  |           0 |        -1 |        0 |          100 |               |            |
(1 row)

//做过了vacuum full的列虽然可以恢复，但是列原来存储的数据已经不存在了，用NULL填充

postgres=# select * from t;
 id | name
----+------
  1 |
  2 |
  3 |
  4 |
  5 |
  6 |
  7 |
  8 |
  9 |
 10 |
(10 rows)