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Re: Sorting Improvements for 8.4

From: Brian Hurt <bhurt(at)janestcapital(dot)com>
To: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Sorting Improvements for 8.4
Date: 2007-12-21 14:50:05
Message-ID: 476BD29D.4070604@janestcapital.com (view raw or flat)
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Lists: pgsql-hackers
Brian Hurt wrote:

> While we're blue skying things, I've had an idea for a sorting 
> algorithm kicking around for a couple of years that might be 
> interesting.  It's a variation on heapsort to make it significantly 
> more block-friendly.  I have no idea if the idea would work, or how 
> well it'd work, but it might be worthwhile kicking around.
>
> Now, the core idea of heapsort is that the array is put into heap 
> order- basically, that a[i] >= a[2i+1] and a[i] >= a[2i+2] (doing the 
> 0-based array version here).  The problem is that, assuming that the 
> length of a is larger than memory, then a[2i+1] is likely going to be 
> on a different page or block than a[i].  That means every time you 
> have to bubble down a new element, you end up reading O(log N) blocks- 
> this is *per element*.
>
> The variation is to instead work with blocks, so you have a block of 
> entries b[i], and you change the definition of heap order, so that 
> min(b[i]) >= max(b[2i+1]) and min(b[i]) >= max(b[2i+2]).  Also, during 
> bubble down, you need to be carefull to only change the minimum value 
> of one of the two child blocks b[2i+1] and b[2i+2].  Other than that, 
> the algorithm works as normal.  The advantage of doing it this way is 
> that while each bubble down still takes O(log N) blocks being touched, 
> you get a entire block worth of results for your effort.  Make your 
> blocks large enough (say, 1/4 the size of workmem) and you greatly 
> reduce N, the number of blocks you have to deal with, and get much 
> better I/O (when you're reading, you're reading megabytes at a shot).
>
> Now, there are boatloads of complexities I'm glossing over here.  This 
> is more of a sketch of the idea.  But it's something to consider.
>
Following up to myself (my apologies), but it's occurred to me that 
there are three advantages to this proposal that I've since thought of:

1) The two child blocks b[2i+1] and b[2i+2]- the one with the larger 
minimum element is the one we might replace.  In other words, if 
min(b[2i+1]) > min(b[2i+2]) and min(b[i]) < min(b[2i+1]), then we know 
we're going to want the blocks b[4i+3] and b[4i+4]- before we're done 
with blocks b[2i+1] and b[2i+2].  The point here is that this would work 
wonders with the posix_fadvise/asyncio ideas kicking around.  It'd be 
easy for the code to keep 2 large writes and 2 large reads going pretty 
constantly.

2) There is some easy parallelization available.  I'm not sure how much 
worth this is, but the bubble down code is fairly easy to parallelize.  
If we have two bubble-downs going on in parallel, once they go down 
different branches (one thread goes to block b[2i+1] while the other 
goes to b[2i+2]) they no longer interact.  Blocks near the root of the 
heap would be contended over, and multiple threads means smaller blocks 
to keep the total memory foot print the same.  Personally, I think the 
asyncio idea above is more likely to be worthwhile.

3) It's possible to perform the sort lazily.  You have the initial O(N) 
pass over the list, but then each block is only O(log N) cost.  If it's 
likely that only the first part of the result is needed, then much of 
the work can be avoided.

Brian


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