As a follow-up to the \if patch by Corey Huinker, here is a proposal to allow testing whether a client-side variable exists in psql.
The syntax is as ugly as the current :'var' and :"var" things, but ISTM that this is the only simple option to have a working SQL-compatible syntax with both client-side substitution and server side execution. See the second example below.
It is really ugly - the ? symbol is not used in pair usually - so it is much more visible - it is bad readable.
Maybe some other syntax: :{fx xxx} .. where fx can be one from more possible operators ? ! ...
-- client side use psql> \set i 1 psql> \if :?i? psql> \echo 'i is defined' psql> \endif
-- use server-side in an SQL expression psql> SELECT NOT :?VERSION_NUM? OR :'VERSION' <> VERSTION() AS bad_conf \gset
The other option would be to have some special keyword syntax, say "defined var", but then it would have to be parsed client side, and how to do that in an SQL expression is unclear, and moreover it would not look right in an SQL expression. If it would look like a function call, say "defined('var')", it would potentially interact with existing server-side user-defined functions, which is pretty undesirable. Hence the :?...? proposal above which is limited to variable subsitution syntax.
