Use a priority queue for the sorted sub-lists. When the key-object extracted from the head of the smallest queue exceeds the key-object from the head of the second queue, adjust the priority of the smallest queue within the list of queues.
It uses a total of 2 read/write passes over the data, no matter how many subfiles you have. It is dominatingly faster than any other sort of external merge when you have lots of subfiles.
I posted some message to the list on this subject before, and gave a pointer to sample code that demonstrates the concept.
If you have one million sub-files, it still only takes a total of two read-write passes.
From: pgsql-hackers-owner@postgresql.org [mailto:pgsql-hackers-owner@postgresql.org] On Behalf Of Jonah H. Harris
Sent: Tuesday, March 07, 2006 5:28 PM
To: Tom Lane
Cc: Simon Riggs; pgsql-hackers@postgresql.org
Subject: Re: [HACKERS] Merge algorithms for large numbers of "tapes"
On 3/7/06, Tom Lane <tgl@sss.pgh.pa.us> wrote:
BTW, I was just looking over Knuth's discussion of sorting again, and
realized that there is still something more that could be done within
the existing sort framework. We currently use standard polyphase merge
(his Algorithm 5.4.2D), which IIRC I chose because it was simple and
for relatively small numbers of tapes T it was about as good as anything
else. Knuth spends a great deal of energy on minimizing tape rewind
time which of course is of no interest to us, and I had supposed that
all of his more-complex algorithms were really only of interest if you
needed to consider rewind time. However, now that we've changed the
code to prefer large numbers of tapes, it's not at all clear that
Algorithm D is still the right one to use. In particular I'm looking at
cascade merge, Algorithm 5.4.3C, which appears to use significantly
fewer passes when T is large. Do you want to try that?
I haven't personally played with this algorithm but having spent the last 15 minutes reading it over, it does sound like an interesting idea for trial. At first glance it didn't seem much better than polyphase for our case, but after reading the entire algorithm, discussion, and thinking it over for a couple minutes, I could see it as potentially better.
Guess we won't really know 'til it can be tested :)
--
Jonah H. Harris, Database Internals Architect
EnterpriseDB Corporation
732.331.1324