Re: Explain Analyze - Total runtime very differentes

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