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Re: Replacement Selection

From: Simon Riggs <simon(at)2ndquadrant(dot)com>
To: mac_man2005(at)hotmail(dot)it
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Replacement Selection
Date: 2007-11-27 12:03:38
Message-ID: 1196165018.4246.950.camel@ebony.site (view raw or flat)
Thread:
Lists: pgsql-hackers
On Tue, 2007-11-27 at 09:25 +0100, mac_man2005(at)hotmail(dot)it wrote:

> Others optimizations, for example, can be done with the "virtual 
> concatenation" technique:
> storing a cache of couples (first_element,last_element) for each created 
> run. This
> could be useful in case we can find 2 couples (first_element_1, 
> last_element_1) and
> (first_element_2, last_element_2) with   last_element_1 <= first_element_2.
> In this case, those runs too can be seen as belonging to the same "logical 
> run"
> (actually they are 2 RS different physical runs, or even 4 in 2WRS
> but can be seen as just one by mergesort). Of course, once those 2 (or 4) 
> runs are
> logically merged into that only one, this last one in turn could be merged 
> to other runs.
> 
> What does all that imply? Mergesort would actually consider a smaller number 
> of runs
> (since it should just work on logical runs). This means less jumps between 
> runs on disk.

That's actually a refinement of an idea I've been working on for
optimizing sort. I'll post those separately.

-- 
  Simon Riggs
  2ndQuadrant  http://www.2ndQuadrant.com


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