包含数据修改语句的WITH子句必须位于顶级SQL状态：0A000

Question

我编写了函数，它使用WITH构造插入到表中，如下所示：

CREATE OR REPLACE FUNCTION test_func()
RETURNS json AS
$BODY$
begin
 return (
   with t as (
     insert into t(id) 
     select 1
     returning *
    )
 select '{"a":"a"}'::json
 );
end;
$BODY$
 LANGUAGE plpgsql VOLATILE;
select test_func()

那是返回错误：

ERROR: WITH clause containing a data-modifying statement must be at the top level
SQL-состояние: 0A000

如果执行

   with t as (
     insert into t(id) 
     select 1
     returning *
    )
 select '{"a":"a"}'::json

结果没有错误。为什么会发生这种情况以及如何绕过这个？

Answer 1

您正在对该查询进行子选择，这就是为什么它不起作用。这也不起作用：

select * from (
   with t as (
     insert into t(id) 
     select 10
     returning *
    )
 select '{"a":"a"}'::json
) as sub

有一些解决方案。

a）声明它返回setof并使用return query

CREATE OR REPLACE FUNCTION test_func()
RETURNS setof json AS
$BODY$
begin
return query
   with t as (
     insert into t(id) 
     select 7
     returning *
    )
 select '{"a":"a"}'::json;
end;
$BODY$
 LANGUAGE plpgsql VOLATILE;

b）将其声明为language sql

CREATE OR REPLACE FUNCTION test_func()
RETURNS json AS
$BODY$
   with t as (
     insert into t(id) 
     select 8
     returning *
    )
 select '{"a":"a"}'::json;
$BODY$
 LANGUAGE sql VOLATILE;

c）在参数列表中声明输出变量并将结果分配给它们

CREATE OR REPLACE FUNCTION test_func(OUT my_out_var json)
AS
$BODY$
begin
   with t as (
     insert into t(id) 
     select 9
     returning *
    )
 select '{"a":"a"}'::json INTO my_out_var;
end;
$BODY$
 LANGUAGE plpgsql VOLATILE;