Re: Inlining comparators as a performance optimisation

Peter Geoghegan wrote:
> Attached are the results from performing a similar process to the
> prior benchmark, but on Greg Smith's high-end server, and with an
> orderlines table that has been "doubled-up" until it is 1538 MB,
> making the same old query perform a quicksort that's over 3GB. Short
> version: HEAD is 20468.0ms, with my patch it's 13689.698ms, so these
> gains hold-up very well for very large in-memory sorts, possibly even
> perfectly.
>
> Note that the "doubling up" had an interesting and desirable effect
> for the purposes of this patch review: It's approaching a worst case
> due to the low cardinality of the product + quantity columns, which
> crops up with the non-inlined functions only (int4regqsort_arg, etc).
> All too frequently, evaluating the first scankey results in equality,
> and so we cannot elide the SQL function call machinery. This is going
> to dampen the improvement for the multiple scanKey case considerably
> (and it looks like a smaller relative improvement than when the
> orderlines table was quite a lot smaller). As I said from the outset,
> there is no worst case for the single scanKey case that occurs to me.
> Multiple scanKeys are likely to be a problem for any effort to offer
> user-defined, per-type sort functions. I could probably make
> int4regqsort_arg and friends perform a bit better than we see here by
> increasing the cardinality of those two columns for the second query,
> so the first scanKey comparison would frequently suffice.

These are exciting advanced you are producing and I am hopeful we can
get this included in Postgres 9.2. I have mentioned already that I
think parallelism is the next big Postgres challenge, and of course, one
of the first areas for parallelism is sorting. If you can speed up
sorting as you have reported, this allows us to provide faster sorting
now, and get even larger benefits if we implement sorting parallelism.

In fact, because Postgres 9.2 has so many performance improvements, we
might find that we have a more limited need for parallelism.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +