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Re: Type Categories for User-Defined Types

From: Dimitri Fontaine <dfontaine(at)hi-media(dot)com>
To: pgsql-hackers(at)postgresql(dot)org
Cc: "David E(dot) Wheeler" <david(at)kineticode(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Subject: Re: Type Categories for User-Defined Types
Date: 2008-07-31 07:49:48
Message-ID: 200807310949.48245.dfontaine@hi-media.com (view raw or flat)
Thread:
Lists: pgsql-hackers
Hi,

Le mercredi 30 juillet 2008, David E. Wheeler a écrit :
> On Jul 30, 2008, at 10:34, Tom Lane wrote:
> >>> [ move preferred-type info into the system catalogs ]
> > UUID and so on aren't considered part of the string category, and
> > shouldn't be IMHO ... any type that has semantics significantly
> > different from "arbitrary string of text" doesn't belong.
>
> Yes, that was essentially my point. "arbitrary string of text" types
> are probably fairly rare, since one can just use text or citext or
> varchar.

I'm not following this thread closely (enough) but my prefix_range type (from 
the pgfoundry prefix module) certainly is a user defined (in C) "arbitrary 
string of text" type. The fact that it's user visible could be questionned, 
its usage is for GiST indexing prefix searches when the prefix is in the 
table, not in the query literal:
  SELECT * FROM prefixes WHERE prefix @> 'literal';

This query currently only profits from the GiST indexing if prefix column is 
of type prefix_range, I'd like to be able to index text and store 
prefix_range in the index, it's on the TODO list (and certainly is doable 
AFAIUI).

I'm not sure this input is valuable for the topic, but would rather not miss 
the example in case it is :)

> > The behavior that's hard-wired into parse_coerce.c at the moment
> > is that only text, varchar, bpchar can be sources or targets of
> > I/O conversions.  While opening it up to citext sounds reasonable,
> > I'm a lot less sure about domains.

I still remember the sour taste of cancelling a domain usage for some type 
needs here after discovering you can't create CAST for domains. Would the 
categorising stuff help here?

Regards,
-- 
dim

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