Re: \if, \elseif, \else, \endif (was Re: [HACKERS] PSQL commands: \quit_if, \quit_unless)

    Corey Huinker wrote:

> Revised patch

A comment about control flow and variables:
in branches that are not taken, variables are expanded
nonetheless, in a way that can be surprising.
Case in point:

\set var 'ab''cd'
-- select :var;
\if false select :var ;
\else select 1;
\endif

The 2nd reference to :var has a quoting hazard, but since it's within
an "\if false" branch, at a glance it seems like this script might work.
In fact it barfs with: psql:script.sql:0: found EOF before closing \endif(s)

AFAICS what happens is that :var gets injected and starts a
runaway string, so that as far as the parser is concerned
the \else ..\endif block slips into the untaken branch, as a part of
that unfinished string.

This contrasts with line 2: -- select :var
where the reference to :var is inoffensive.

To avoid that kind of trouble, would it make sense not to expand
variables in untaken branches?

Best regards,
--
Daniel Vérité
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