Subject: Re: PostgreSQL vs SQL/XML Standards
To: pgsql-hackers@lists.postgresql.org
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From: Chapman Flack <chap@anastigmatix.net>
Message-ID: <5C4401BE.4080606@anastigmatix.net>
Date: Sun, 20 Jan 2019 00:06:06 -0500
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On 01/19/19 23:49, Pavel Stehule wrote:

> If I remember, described functionality was implemented in early patches,
> but was removed to simplify code. To now, there was not a request to do it.
> 
> Unfortunately, the documentation was not fixed.

I'll do that, as I'm working in there anyway. :)

> column expressions are evaluated once per row, but XPath  expression is
> compiled per row too, if I remember well.

I looked for evidence of that in the code, but did not find it; the
compilation appears to happen in XmlTableSetColumnFilter, which is
called from tfuncInitialize.

I can't guarantee I didn't miss something, though.

> We designed it more tolerant as
> we expected possibility to store XPath expression in one column and data in
> second column.

Perhaps if I could see an example showing the functionality... The nearest
I could find in the regression tests was the test with
 COLUMNS a int PATH '' || lower(_path) || 'c'
but, again, I think that test only demonstrates how LATERAL works, not
any behavior specific to xmltable.

Regards,
-Chap