Re: Selecting a constant question

> -----Original Message-----
> From: Gregory Stark [mailto:stark@enterprisedb.com]
> Sent: Monday, June 11, 2007 12:48 PM
> To: Dann Corbit
> Cc: pgsql-hackers@postgresql.org
> Subject: Re: [HACKERS] Selecting a constant question
>
> "Dann Corbit" <DCorbit@connx.com> writes:
>
> > SELECT 1  FROM test.dbo.a_003
> >
> > gets about 60,000 records per second
> >
> > SELECT '1'  FROM test.dbo.a_003
> >
> > gets about 600 records per second.
> >
> > The cause is that postgres describes the return column as "unknown"
> > length 65534 in the 2nd case.
>
> Wait, back up. How does this cause it to go slower?

The issue is this:

Postgres describes the column with a typmod of -1 (unknown) and a length
of 65534.

This means that any client application must reserve 65534 bytes of
spaces for every row of data (like a grid control for example), which
postgres should know (and report) that the maximum length of the column
is 1.

This is not a PSQL issue, it's an issue with other products relying on
the accuracy of the reported postgres metadata for a given SQL
statement.