Re: patch: function xmltable

From: Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>
To: Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
Cc: PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>, Craig Ringer <craig(at)2ndquadrant(dot)com>
Subject: Re: patch: function xmltable
Date: 2017-03-02 21:35:44
Message-ID: 20170302213544.ml3lgkip4rwirjnd@alvherre.pgsql
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Pavel Stehule wrote:
> 2017-03-02 19:32 GMT+01:00 Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>:
> > So in the old (non-executor-node) implementation, you could attach WITH
> > ORDINALITY to the xmltable expression and it would count the output
> > rows, regardless of which XML document it comes from. With the new
> > implementation, the grammar no longer accepts it. To count output rows,
> > you still need to use row_number(). Maybe this is okay. This is the
> > example from the docs, and I add another XML document with two more rows
> > for xmltable. Look at the three numbering columns ...
> >
> It is expected - now tablefunc are not special case of SRF, so it lost all
> SRF functionality. It is not critical lost - it supports internally FOR
> ORDINALITY column, and classic ROW_NUMBER can be used. It can be enhanced
> to support WITH ORDINALITY in future, but I have not any use case for it.


After looking at the new executor code a bit, I noticed that we don't
need the resultSlot anymore; we can use the ss_ScanTupleSlot instead.
Because resultSlot was being used in the xml.c code (which already
appeared a bit dubious to me), I changed the interface so that instead
the things that it read from it are passed as parameters -- namely, in
InitBuilder we pass natts, and in GetValue we pass typid and typmod.

Secondly, I noticed we have the FetchRow routine produce a minimal
tuple, put it in a slot; then its caller takes the slot and put the
tuple in the tuplestore. This is pointless; we can just have FetchRow
put the tuple in the tuplestore directly and not bother with any slot
manipulations there. This simplifies the code a bit.

Here's v47 with those changes.

Álvaro Herrera
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Attachment Content-Type Size
xmltable-47.patch.gz application/x-gunzip 36.8 KB

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