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Re: xpath processing brain dead

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Andrew Dunstan <andrew(at)dunslane(dot)net>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Peter Eisentraut <peter_e(at)gmx(dot)net>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Nikolay Samokhvalov <samokhvalov(at)gmail(dot)com>
Subject: Re: xpath processing brain dead
Date: 2009-02-27 22:02:55
Message-ID: 24524.1235772175@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Andrew Dunstan <andrew(at)dunslane(dot)net> writes:
>> I'll do some tests to see what the cost of extra xml parsing might be.

> The extra cost appears to be fairly negligible.

Uh, you didn't actually show a comparison of before and after?
What it looks like to me is that this approach is free or nearly so for
well-formed documents, but doubles the parsing cost for forests.
Which is likely to annoy anyone who's really depending on the
capability.

Also,

> ! 		if (*VARDATA(xpath_expr_text) == '/')

This is risking a core dump if the xpath expr is of zero length.  You
need something like

		if (xpath_len > 0 && *VARDATA(xpath_expr_text) == '/')

It would also be a good idea if the allocation of string and xpath_expr
had a comment about why it's allocating extra space (something like "see
hacks below for use of this extra space" would be sufficient).

			regards, tom lane

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