将列类型从 char 更改为 array

Question

我在 PostgreSQL 中有一个 table，它有一个 character(1) 列，我想将其更改为 text[] 列，但我似乎无法让它工作：

ALTER TABLE public.mytable
     ALTER COLUMN abc TYPE TEXT[] COLLATE pg_catalog."default"
     USING ARRAY[abc];

给我这个错误：

ERROR: default for column "abc" cannot be cast automatically to type text[]

这是可以理解的，因为 Postgres 无法将 NULL:bpchar 转换为数组。但是，我该如何完成呢？显然，可以输入 NULLs ...

Answer 1

您需要删除默认值，更改数据类型并重新添加默认值。
来自 documentation:

(…) the USING expression is not applied to the column's default value (if any); (…).

This means that when there is no implicit or assignment cast from old to new type, SET DATA TYPE might fail to convert the default even though a USING clause is supplied. In such cases, drop the default with DROP DEFAULT, perform the ALTER TYPE, and then use SET DEFAULT to add a suitable new default.

和给定的例子：

> When the column has a default expression that won't automatically cast to the 
  new data type:

ALTER TABLE foo
    ALTER COLUMN foo_timestamp DROP DEFAULT,
    ALTER COLUMN foo_timestamp TYPE timestamp with time zone
    USING timestamp with time zone 'epoch' + foo_timestamp * interval '1 second',
    ALTER COLUMN foo_timestamp SET DEFAULT now();

将列类型从 char 更改为 array

Changing a column type from char to array

postgresql

alter