使用 plpgsql 变量设置 n_distinct 时出错

Question

我尝试使用函数为 table 设置 n_distinct 值。代码如下：

create temporary table _temp
(
  id integer
);

create function pg_temp.setdistinct(_cnt real)
returns void as $$
begin
  alter table _temp
    alter column id set (n_distinct=_cnt);
end
$$ language plpgsql;

select pg_temp.setdistinct(1000);

但收到以下错误：

ERROR:  invalid value for floating point option "n_distinct": _cnt
CONTEXT:  SQL statement "alter table _temp
         alter column id set (n_distinct=_cnt)"
PL/pgSQL function pg_temp_3.setdistinct(real) line 3 at SQL statement

这个问题可以使用 EXECUTE 语句绕过，但我想知道为什么我们不能在这个特定的查询中使用变量。有没有我忽略的特定规则？

Answer 1

这是设计使然。手册在章节 Variable Substitution:

中解释

Variable substitution currently works only in SELECT, INSERT, UPDATE, and DELETE commands, because the main SQL engine allows query parameters only in these commands. To use a non-constant name or value in other statement types (generically called utility statements), you must construct the utility statement as a string and EXECUTE it.

您不能使用 EXECUTE 参数化动态语句中的值，因为 quoting the chapter Executing Dynamic Commands:

Another restriction on parameter symbols is that they only work in SELECT, INSERT, UPDATE, and DELETE commands. In other statement types (generically called utility statements), you must insert values textually even if they are just data values.

plpgsql 函数中的唯一选项 是将值连接到命令字符串中。您可能会使用 format()，但对于简单的示例，普通连接是安全且简单的::

CREATE OR REPLACE FUNCTION pg_temp.setdistinct(_cnt real)
  RETURNS void
  LANGUAGE plpgsql AS
$$
BEGIN
   EXECUTE 'ALTER TABLE _temp ALTER COLUMN id SET (n_distinct=' || _cnt || ')';
END
$$;

模式限定 pg_temp. 使其成为（未记录！）“临时”函数，反映了问题。
关于n_distinct.

的手册

使用 plpgsql 变量设置 n_distinct 时出错

Error when setting n_distinct using a plpgsql variable

postgresql

alter-table

dynamic-sql

plpgsql