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Re: Native XML

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Andrew Dunstan <andrew(at)dunslane(dot)net>, Anton <antonin(dot)houska(at)gmail(dot)com>, Peter Eisentraut <peter_e(at)gmx(dot)net>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Native XML
Date: 2011-02-28 16:23:10
Message-ID: AANLkTi=XmKZEHaq9VoJ9WUSnL1Gr8kapSmoJhUC6AV_e@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Mon, Feb 28, 2011 at 10:30 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Well, in principle we could allow them to work on both, just the same
> way that (for instance) "+" is a standardized operator but works on more
> than one datatype.  But I agree that the prospect of two parallel types
> with essentially duplicate functionality isn't pleasing at all.

The real issue here is whether we want to store XML as text (as we do
now) or as some predigested form which would make "output the whole
thing" slower but speed up things like xpath lookups.  We had the same
issue with JSON, and due to the uncertainty about which way to go with
it we ended up integrating nothing into core at all.  It's really not
clear that there is one way of doing this that is right for all use
cases.  If you are storing xml in an xml column just to get it
validated, and doing no processing in the DB, then you'd probably
prefer our current representation.  If you want to build functional
indexes on xpath expressions, and then run queries that extract data
using other xpath expressions, you would probably prefer the other
representation.

I tend to think that it would be useful to have both text and
predigested types for both XML and JSON, but I am not too eager to
begin integrating more stuff into core or contrib until it spends some
time on pgfoundry or github or wherever people publish their
PostgreSQL extensions these days and we have a few users prepared to
testify to its awesomeness.

In any case, the definitional problems with xpath_table(), and/or the
memory management problems with libxml2, are not the basis on which we
should be making this decision.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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Subject: Re: Native XML
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