"David G. Johnston" <david.g.johns...@gmail.com> writes:
> On Mon, Jan 23, 2017 at 11:16 AM, Fabien COELHO <coe...@cri.ensmp.fr> wrote:
>> Currently the value of a non existing psql-variable is... its own
>> reference:-(
>> psql> \echo :x
>> :x
>> I'm not sure of the rational, apart from the probable lexer implementation
>> point of view. Maybe an empty string or 0 or some configurable value would
>> provide better alternative.

> The fundamental problem is that:
> SELECT 'testing' AS ":tablename"
> is perfectly valid SQL code.

Yeah, but psql does know not to try to resolve :something inside a quoted
literal or identifier.  The actual problem is with constructs like

        SELECT somearray[lower:upper] FROM ...

If the user is thinking that's an array subscript not a variable
reference, we don't want to break their query when we don't even have
a useful thing to contribute.

Back in the day, PG allowed ":" as a generic operator name, making
this even worse; but I think the only remaining SQL syntax that could
include a colon is array slicing.

                        regards, tom lane

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