> -----Original Message----- > From: Tom Lane [mailto:[EMAIL PROTECTED] > Sent: Monday, June 11, 2007 5:32 PM > To: Larry McGhaw > Cc: Alvaro Herrera; Dann Corbit; Gregory Stark; Martijn van Oosterhout; > pgsql-hackers@postgresql.org > Subject: Re: [HACKERS] Selecting a constant question > > "Larry McGhaw" <[EMAIL PROTECTED]> 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. ---------------------------(end of broadcast)--------------------------- TIP 6: explain analyze is your friend
