From: Noam Rathaus (noamr_at_nonexisting.hamakor.org.il)
Date: Thu 17 Jun 2004 - 13:50:27 IDT
On Wednesday 16 June 2004 21:12, Omer Zak wrote:
> On Wednesday 16 June 2004 12:14, Yuval Yaari wrote:
> > Hi,
> >
> > I had a query that used JOIN and took too much time (4 seconds).
> > I figured Perl could handle it much faster, and so it did.
>
> While Perl may be doing it faster, I'd suggest that you check why your
> JOIN took excessively long time. Properly defined tables and JOINs should
> execute faster in MySQL, when optimized, than in Perl. Maybe you were
> missing an index, or used suboptimal join order?
>
> To investigate this, you may want to use the EXPLAIN SELECT command. This
> command causes MySQL to output a table telling you how it would actually
> execute the SELECT command. You'll see whether it needs to examine
> excessively large number of rows in the tables, and whether adding an
> index can improve SELECT performance while not degrading too much
> INSERT/DELETE performance.
>
> --- Omer
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>
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Hi,
I don't know from where this email came, if I had the original email I could
have maybe seen if there was a mistake in the query. In any case MySQL does
optimizations to the queries, and these optimizations aren't always good, for
example for UNION and INNER JOINS, so if your Perl does it quicker its only
because the query is probably not written correctly.
One good example is not to use UNION and INNER JOIN too many times in the same
query, but rather place the results inside a temporary HEAP TABLE you create
(in SQL syntax there is support for it), and then requery this HEAP table.
-- Thanks Noam Rathaus CTO Beyond Security Ltd. Join the SecuriTeam community on Orkut: http://www.orkut.com/Community.aspx?cmm=44441 ================================================================= To unsubscribe, send mail to linux-il-request_at_linux.org.il with the word "unsubscribe" in the message body, e.g., run the command echo unsubscribe | mail linux-il-request_at_linux.org.il
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