Message-ID: <48FAF616.4040005@kaltenbrunner.cc>
Date: Sun, 19 Oct 2008 10:55:50 +0200
From: Stefan Kaltenbrunner <stefan@kaltenbrunner.cc>
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To: tarcizioab@c3sl.ufpr.br
CC: pgsql-performance@postgresql.org
Subject: Re: Explain Analyze - Total runtime very differentes
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tarcizioab@c3sl.ufpr.br wrote:
> Hello friends ...
> 
> I'm evaluating the performance of algorithms for optimization of queries.
> I am comparing results between the algorithm of Dynamic Programming and an
> implementation of Kruskal's algorithm. When submitting a query that makes
> reference to only 2 tables of my base, logically the same "Query Plan" is
> shown. But the "Total runtime" displayed by the command "Explain-Analyze"
> presents a variation of time very high:
> 
> Dynamic Programming Total runtime: 1204.220 ms
> 
> Kruskal Total runtime: 3744.879 ms
> 
> No change of data (insert, delete, update) in the tables was made during
> the tests. The same query was submitted several times (with Kruskal and
> Dynamic Programming algorithms) and the variation of results persists.
> 
> The "explain analyze" only reports the time to run *execute* the query.
> With the same "Query Plan", does not understand why this variation occurs.
> 
> In annex the Query Plans

sure it it not something as simple as a caching effect - ie you run the 
slow variant first and pg and/or the OS buffered data and the repeated 
execution just got a benefit from that ?

Try running all variations a few dozend times both in cached and 
uncached state and you should see the difference getting leveled out.


Stefan