I was giving some thought to how psql handles undefined variables.
I would like an option where either psql can provide an alternate value when an undefined variable is referenced, or a way to detect that a specific variable is undefined and replace it with a defined variable.
My first thought thought was to have a
\set_if_undefined var_name 'default_value'
\coalesce var_name [maybe other var names?] 'default_value' --sets the value of var_name to itself or the first defined/non-null result of the subsequent items. Probably only useful if you allow expressions. I have followed the "\if" thread that closely but IIRC that was part of the discussion there.
Another idea adding a \pset parameter that would return a specific value when an undefined psql variable is referenced instead of raising an error. Like this:
# select :'x' as value_of_x;
ERROR: syntax error at or near ":"
LINE 1: select :'x' as value_of_x;
^
# \pset variable_default ''
analytics=# select :'x' as value_of_x;
value_of_x
------------
(1 row)
-1
I unconvinced that setting this in the global environment is a good thing.
