Re: [PERFORM] A Better External Sort? - Mailing list pgsql-hackers

*blink* Tapes?!  I thought that was a typo...
If our sort is code based on sorting tapes, we've made a mistake.  HDs
are not tapes, and Polyphase Merge Sort and it's brethren are not the
best choices for HD based sorts.

Useful references to this point:
Knuth, Vol 3 section 5.4.9, (starts p356 of 2ed)
Tharp, ISBN 0-471-60521-2, starting p352
Folk, Zoellick, and Riccardi, ISBN 0-201-87401-6, chapter 8 (starts p289)

The winners of the "Daytona" version of Jim Gray's sorting contest, for
general purpose external sorting algorithms that are of high enough quality
to be offered commercially, also demonstrate a number of better ways to
attack external sorting using HDs.

The big take aways from all this are:
1= As in Polyphase Merge Sort, optimum External HD Merge Sort
performance is obtained by using Replacement Selection and creating
buffers of different lengths for later merging.  The values are different.

2= Using multiple HDs split into different functions, IOW _not_ simply
as RAIDs, is a big win.
A big enough win that we should probably consider having a config option
to pg that allows the use of HD(s) or RAID set(s) dedicated as temporary
work area(s).

3= If the Key is small compared record size, Radix or Distribution
Counting based algorithms are worth considering.

The good news is all this means it's easy to demonstrate that we can
improve the performance of our sorting functionality.

Assuming we get the abyssmal physical IO performance fixed...
(because until we do, _nothing_ is going to help us as much)

Ron


-----Original Message-----
From: Tom Lane <tgl@sss.pgh.pa.us>
Sent: Oct 1, 2005 2:01 AM
Subject: Re: [HACKERS] [PERFORM] A Better External Sort?

"Jeffrey W. Baker" <jwbaker@acm.org> writes:
> I think the largest speedup will be to dump the multiphase merge and
> merge all tapes in one pass, no matter how large M.  Currently M is
> capped at 6, so a sort of 60GB with 1GB sort memory needs 13 passes over
> the tape.  It could be done in a single pass heap merge with N*log(M)
> comparisons, and, more importantly, far less input and output.

I had more or less despaired of this thread yielding any usable ideas
:-( but I think you have one here.  The reason the current code uses a
six-way merge is that Knuth's figure 70 (p. 273 of volume 3 first
edition) shows that there's not much incremental gain from using more
tapes ... if you are in the regime where number of runs is much greater
than number of tape drives.  But if you can stay in the regime where
only one merge pass is needed, that is obviously a win.

I don't believe we can simply legislate that there be only one merge
pass.  That would mean that, if we end up with N runs after the initial
run-forming phase, we need to fit N tuples in memory --- no matter how
large N is, or how small work_mem is.  But it seems like a good idea to
try to use an N-way merge where N is as large as work_mem will allow.
We'd not have to decide on the value of N until after we've completed
the run-forming phase, at which time we've already seen every tuple
once, and so we can compute a safe value for N as work_mem divided by
largest_tuple_size.  (Tape I/O buffers would have to be counted too
of course.)

It's been a good while since I looked at the sort code, and so I don't
recall if there are any fundamental reasons for having a compile-time-
constant value of the merge order rather than choosing it at runtime.
My guess is that any inefficiencies added by making it variable would
be well repaid by the potential savings in I/O.