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'
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)
# \pset variable_default ''
# select :'x' as value_of_x;
ERROR: syntax error at or near ":"
LINE 1: select :'x' as value_of_x;
^
This would end up having behavior somewhat similar to +e/-e in bash, where a paranoid script could do something like this:
\pset variable_default 'default1'
select :'required_var1' as required_var1 \gset
\pset variable_default 'default2'
select :'required_var2' as required_var2 \gset
-- reset to default behavior
\pset variable_default error
Thus setting sane defaults to vars that weren't assigned at invocation time.