1. :xx is out of SQL syntax, so can by safely used. There is not risk of unwanted usage.
But there is absence of wanted usage too.
How much strong and often? The parser of SQL in psql is not nice - and I understand so nobody would to complicate syntax. Current design is SAFE and good enough. The problem is not is a evaluation, but in DO implementation.
2. but string literal can contain :xxx symbols and not necessary it means so it should be usage of psql variable - so additional syntax for disabling evaluation should be necessary
Yes and Oracle sqlplus (I khow than you know this tool) has special command to control this: set define ...
Unfortunately DO command is half baked - and doesn't support parameters. I am working on schema variables and I hope it will be a solution of this issue:
CREATE VARIABLE var as integer;
LET var = :psqlintvar;
DO $$
BEGIN
RAISE NOTICE '%', var;
END; $$;
It will be great. I already commented it in your blog.
I am slowly working on prototype. The work is simple, when variables are just scalars. But it is much harder, when we allow composite variables. When prototype will be done, I invite any cooperation - there are lot of question - and one very hard - where and how the variable should be stored (values) - now I have workaround, but it is pretty ugly code.
CREATE VARIABLE x INT;
LET x = 10;
BEGIN;
DROP VARIABLE x;
ROLLBACK;
SELECT x; -- should be 10 .. for this situation, the PostgreSQL internal caches are not prepared