本节简单介绍了PostgreSQL 12中的generated columns.在其他数据库中,generated columns又被成为计算列(calculated columns)/虚拟列(virtual columns)等.

**简介**
在PG 11或以前的版本中,generated columns是不支持的:
```
testdb=# drop table if exists t_generated_col;
NOTICE:  table "t_generated_col" does not exist, skipping
DROP TABLE
testdb=# create table t_generated_col
testdb-# (n1 int,
testdb(#  n2 int,
testdb(#  c1 varchar(10),
testdb(#  c2 varchar(10),
testdb(#  counter int generated always as (n1 + n2) stored,
testdb(#  link varchar(20) generated always as (c1 || c2) stored);
ERROR:  syntax error at or near "("
LINE 6:  counter int generated always as (n1 + n2) stored,
                                         ^
```
counter列的值由n1和n2相加而得,而link列的值有c1和c2拼接而得.
在PG 12中,可以支持generated columns
```
testdb=# drop table if exists t_generated_col;
psql: NOTICE:  table "t_generated_col" does not exist, skipping
DROP TABLE
testdb=# create table t_generated_col
testdb-# (n1 int,
testdb(#  n2 int,
testdb(#  c1 varchar(10),
testdb(#  c2 varchar(10),
testdb(#  counter int generated always as (n1 + n2) stored,
testdb(#  link varchar(20) generated always as (c1 || c2) stored);
CREATE TABLE
testdb=# insert into  t_generated_col(n1,n2,c1,c2) values(1,1,'c1','c2');
INSERT 0 1
testdb=# 
testdb=# select * from t_generated_col;
 n1 | n2 | c1 | c2 | counter | link 
----+----+----+----+---------+------
  1 |  1 | c1 | c2 |       2 | c1c2
(1 row)

```
生成列的值由表达式的值计算而得,如为null则为null:
```
testdb=# insert into  t_generated_col(n1,n2,c1,c2) values(1,null,'c1',null);
INSERT 0 1
testdb=# select * from t_generated_col;
 n1 | n2 | c1 | c2 | counter | link 
----+----+----+----+---------+------
  1 |  1 | c1 | c2 |       2 | c1c2
  1 |    | c1 |    |         | 
(2 rows)
```
生成列不能被update:
```
testdb=# update t_generated_col set counter = 10;
psql: ERROR:  column "counter" can only be updated to DEFAULT
DETAIL:  Column "counter" is a generated column.
```
可以创建在其上创建index:
```
testdb=# create index idx_t_generated_col_counter on t_generated_col(counter);
CREATE INDEX
```
执行查询时,跟普通列没有什么区别:
```
                                                  ^
testdb=# insert into  t_generated_col(n1,n2) select x,0 from generate_series(1,10000) x;
INSERT 0 10000
testdb=# 
testdb=# explain select * from t_generated_col where counter = 1000;
                                             QUERY PLAN                                             
----------------------------------------------------------------------------------------------------
 Index Scan using idx_t_generated_col_counter on t_generated_col  (cost=0.29..8.30 rows=1 width=23)
   Index Cond: (counter = 1000)
(2 rows)
```

**参考资料**
[Generated columns in PostgreSQL 12](https://www.2ndquadrant.com/en/blog/generated-columns-in-postgresql-12/)

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PostgreSQL DBA(54) - PG 12 Generated columns

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