proposal: plpgsql, new check for extra_errors - strict_expr_check

Hi,

assigned patch try to solve issue reported by Mor Lehr (Missing semicolon
in anonymous plpgsql block does not raise syntax error).

https://www.postgresql.org/message-id/CALyvM2bp_CXMH_Gyq87pmHJRuZDEhV40u9VP8rX=CAnEt2wUXg@mail.gmail.com

by introducing a new extra error check. With this check only a_expr exprs
are allowed as plpgsql expressions. This is a small step to behaviour
described in SQL/PSM standard (although the language is different, the
expression syntax and features are almost similar. With this check the
undocumented (but supported syntax)

var := column FROM tab

is disallowed. Only ANSI syntax for embedded queries (inside assignment
statement) is allowed

var := (SELECT column FROM tab);

With this check, the reported issue (by Mor Lehr) is detected

default setting

CREATE TABLE foo3(id serial PRIMARY key, txt text);
INSERT INTO foo3 (txt) VALUES ('aaa'),('bbb');

DO $$
DECLARE
l_cnt int;
BEGIN
l_cnt := 1
DELETE FROM foo3 WHERE id=1;
END; $$

-- without reaction - just don't work

(2024-06-16 16:05:55) postgres=# set plpgsql.extra_errors to
'strict_expr_check';
SET
(2024-06-16 16:06:43) postgres=# DO $$

DECLARE
l_cnt int;
BEGIN
l_cnt := 1
DELETE FROM foo3 WHERE id=1;
END; $$;
ERROR: syntax error at or near "DELETE"
LINE 11: DELETE FROM foo3 WHERE id=1;
^

This patch has three parts

1. Introduction strict_expr_check
2. set strict_expr_check as default, and impact on regress tests
3. revert @2

I don't propose to be strict_expr_check active by default.

Comments, notes?

Regards

Pavel