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

Question

I wrote function, which using WITH construction with insert into table like this:
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()
Thats return error:
ERROR: WITH clause containing a data-modifying statement must be at the top level
SQL-состояние: 0A000
If execute
with t as (
insert into t(id)
select 1
returning *
)
select '{"a":"a"}'::json
Result without errors.
Why this take place and how get round this?

Answer 1

You are doing subselect on that query, with is why it doesn't work.
This won't work either:
select * from (
with t as (
insert into t(id)
select 10
returning *
)
select '{"a":"a"}'::json
) as sub
There are a few solutions to this.
a) Declare it as returning setof and use 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) Declare it as 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) Declare output variable(s) in argument list and assign result to them
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;