On Fri, Nov 29, 2019 at 05:19:00AM +0000, Andrew Gierth wrote:
"Alvaro" == Alvaro Herrera <alvhe...@2ndquadrant.com> writes:
Alvaro> First, I noticed that there's a significant unanswered issue
Alvaro> from Andrew Gierth about this using a completely different
Alvaro> mechanism, namely an implicit window function. Robert was
Alvaro> concerned about the performance of Andrew's proposed approach,
Alvaro> but I don't see any reply from Surafel on the whole issue.
Alvaro> Andrew: Would you rather not have this feature in this form at
Alvaro> all?
I have done a proof-of-concept hack for my alternative approach, which I
will post in another thread so as not to confuse the cfbot.
The main advantage, as I see it, of a window function approach is that
it can also work for PERCENT with only a change to the generated
expression; the executor part of the code can handle any stopping
condition. It can also be generalized (though the POC does not do this)
to allow an SQL-level extension, something like "LIMIT WHEN condition"
to indicate that it should stop just before the first row for which the
condition is true. Whether this is a desirable feature or not is another
question.
For the record, the alternative approach was posted here:
https://www.postgresql.org/message-id/flat/87o8wvz253....@news-spur.riddles.org.uk
I think we need to make a decision about which road to take - shall we
continue with what Surafel originally proposed, or should we instead
adopt Andrew's patch?
Andrew's patch looks significantly simpler, although it probably will
need more work to get it committable. My main concern about using window
functions was performance, so I think we should benchmark. For example,
considering window functions are not parallel safe, would this have
negative impact on parallel queries?
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
