Re: Selecting a constant question

> -----Original Message-----
> From: Tom Lane [mailto:tgl(at)sss(dot)pgh(dot)pa(dot)us]
> Sent: Monday, June 11, 2007 5:32 PM
> To: Larry McGhaw
> Cc: Alvaro Herrera; Dann Corbit; Gregory Stark; Martijn van
Oosterhout;
> pgsql-hackers(at)postgresql(dot)org
> Subject: Re: [HACKERS] Selecting a constant question
>
> "Larry McGhaw" <lmcghaw(at)connx(dot)com> writes:
> > I think perhaps we have lost sight of the main issue:
> > 1) libpq can properly describe the maximum internal data size of any
> > numeric or char column in a table via Pqfsize
> > 2) libpq can properly describe the maximum internal data size of any
> > varchar column via Pqfmod
> > 3) libpq can properly describe the maximum internal data size of any
> > numeric constant in a SQL statement via Pqfsize
>
> None of the above statements are actually true, at least not when you
> take off your blinders and note the existence of unconstrained-width
> numeric and text columns.

Unconstrained width columns are not what are being discussed here. It
is constant expressions of known width.

> > The database *knows* this size of the char constant (obviously),
>
> No, what it knows (and reports) is type information. There are a
small
> number of datatypes where you can infer a maximum width from knowledge
> of the datatype. There are many others where you can't set an upper
> bound from this knowledge --- at least not a usefully tight one.

If you do not know how large 1::numeric is, then how can you know
whether it is safe or not to insert it into a column of type
numeric(12,4)?

If you do not know how large 'Joe'::varchar is, then how can you know
whether it is safe to insert it into a column of type varchar(256)?

Clearly, neither of these operations will cause any problems and so the
size of a constant can be determined.

> Anyway, if we were to cast those constants to something other than
> unknown, it would be text, not varchar, and you'd still have the same
> issue.

Other database systems can manage this, and the programmers of those
database systems are not smarter than the programmers of the PostgreSQL
group. Therefore I can conclude that if the PostgreSQL group decides it
is important, then they can figure out the size of a string or numeric
constant.