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Re: patch: function xmltable

From: Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>
To: Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
Cc: Craig Ringer <craig(at)2ndquadrant(dot)com>,PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: patch: function xmltable
Date: 2017-01-24 20:38:49
Message-ID: 20170124203849.2r2xzkraix6qy3i7@alvherre.pgsql (view raw, whole thread or download thread mbox)
Thread:
Lists: pgsql-hackers
Pavel Stehule wrote:

> * SELECT (xmltable(..)).* + regress tests
> * compilation and regress tests without --with-libxml

Thanks.  I just realized that this is doing more work than necessary --
I think it would be simpler to have tableexpr fill a tuplestore with the
results, instead of just expecting function execution to apply
ExecEvalExpr over and over to obtain the results.  So evaluating a
tableexpr returns just the tuplestore, which function evaluation can
return as-is.  That code doesn't use the value-per-call interface
anyway.

I also realized that the expr context callback is not called if there's
an error, which leaves us without shutting down libxml properly.  I
added PG_TRY around the fetchrow calls, but I'm not sure that's correct
either, because there could be an error raised in other parts of the
code, after we've already emitted a few rows (for example out of
memory).  I think the right way is to have PG_TRY around the execution
of the whole thing rather than just row at a time; and the tuplestore
mechanism helps us with that.

I think it would be good to have a more complex test case in regress --
let's say there is a table with some simple XML values, then we use
XMLFOREST (or maybe one of the table_to_xml functions) to generate a
large document, and then XMLTABLE uses that document as input document.

Please fix.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Attachment: xmltable-36.patch
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