Re: Native XML

On Wed, Mar 9, 2011 at 1:11 PM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> Robert Haas wrote:
>> 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.
>
> Someone should measure how much overhead the indexing of xml values
> might have.  If it is minor, we might be OK with only an indexed xml
> type.

I think the relevant thing to measure would be how fast the
predigested representation speeds up the evaluation of xpath
expressions.

--
Robert Haas
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