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Unnest() and xml

From: "Tamayo, Damian-p65828" <Damian(dot)Tamayo(at)gdc4s(dot)com>
To: <pgsql-in-general(at)postgresql(dot)org>
Subject: Unnest() and xml
Date: 2009-07-08 15:43:12
Message-ID: 2B0195B08BA41748A38C4D58ADF44F2C0385C3E6@AZ25EXM01.gddsi.com (view raw or flat)
Thread:
Lists: pgsql-in-general
So I have general question.... I have searched through the archives and
have yet to find a clear cut explanation for this....

When xpath() function is performed....

Can someone tell me whether or not the function creates an array on the
fly or does it create an array of the size of the table and then fill in
the slots depending on whether or not the row is valid??

Xml
Xml
{}
{}
Xml

So when run... the above column would get returned as such.... Is this a
predetermined array? Or is postgres simply returning the empty values to
me because it thinks that I need them?

Also I notice that when I use the unnest function the empty slots are
removed....hence:

Xml
Xml
Xml

Are returned to me....while I would to use this implementation... I'm
curious as to how the internals work here... if it creates the array of
a size(size = # of rows) before hand then I might need to use a
different implementation than what I'm currently using due to efficiency
problems that may arise later on when the tables are quite large....

Also If I have posted to the wrong mailing list ... please let me know.
Thank you for your time.

> Damian Tamayo
> General Dynamics C4 Systems
> 
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