Re: Window functions patch v04 for the September commit fest - Mailing list pgsql-hackers

On Sat, 2008-08-30 at 02:04 +0900, Hitoshi Harada wrote:

> Here's the latest window functions patch against HEAD. It seems to be
> ready for the September commit fest, as added documents, WINDOW clause
> feature and misc tests. I guess this would be the window functions
> feature freeze for 8.4. The remaining feature will be implemented for
> the later release.
> 
> This patch consists of:
> - windowed aggregates
> - cooperate with GROUP BY aggregates
> - some optimizations with multiple windows
> - ranking functions
> - WINDOW clause
> - windowing SQL regression tests
> - sgml documents
> 
> Looking up the total road map, the dropped features are:
> 
> - sliding window (window framing)
> - lead(), lag(), etc. that reach for random rows
> - user defined window functions
> 
> The first and second topics are difficult to implement currently.
> Because these features require random row access, it seems that
> tuplestore would be able to save multiple positions to mark/restore.
> This is fundamental change that is over my capability. Also, user
> defined window functions seem to have much more to decide. I think I
> can't put into shape the general needs of user's window functions now.
> Lacking these feature, this stage looks compatible to SQLServer 2005,
> while Oracle and DB2 have almost full of the specification.

If you've done all of that, then I'm impressed. Well done.

Few general comments

* The docs talk about "windowing functions", yet you talk about "window
functions" here. I think the latter is correct, but whichever we choose
we should be consistent (and hopefully matching SQL Standard).

* You don't use duplicate the examples from the docs into the tests,
which is always a good way to get conflicting reports from users. :-)

* The tests seem very light for such a huge range of new functionality.
(8 tests is hardly sufficient). I'd like to see a wide range of tests -
probably 5-10 times as many individual test statements. I would also
like to see test failures that illustrate the as-yet unimplemented
features and the warning messages that are thrown - this will help us
understand exactly what is missing also. It would also be useful to see
other common coding mistakes/misconceptions and the corresponding error
messages.

> Also, current implementation has only a type of plan which uses sort
> operation. It should be optimized by re-position the windows and/or
> using hashtable.

I would like to see some performance test results also. It would be good
to know whether they are fast/slow etc.. It will definitely help the
case for inclusion if they are faster than alternative multi-statement
approaches to solving the basic data access problems.

-- Simon Riggs           www.2ndQuadrant.comPostgreSQL Training, Services and Support