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From "Knut Anders Hatlen (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-4971) Query runs 10 times more slowly with optimizer turned ON
Date Fri, 14 Jan 2011 14:00:58 GMT

    [ https://issues.apache.org/jira/browse/DERBY-4971?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12981763#action_12981763

Knut Anders Hatlen commented on DERBY-4971:

With 12 tables being joined, there are 12! == 479,001,600 possible join orders, so it's not
completely unreasonable that it takes longer when the optimizer must pick the join order itself.
There is a timeout mechanism to prevent the optimizer from spending a ridiculous amount of
time trying to improve a plan that's most likely good enough, but because of bugs like DERBY-1905
it doesn't always work properly.

It may look like something similar to DERBY-1905 is what comes into play here. The cost estimates
do indeed look inflated. The estimated cost for the chosen query plans is ~44 seconds, which
fits well with the 45 seconds spent preparing the statement (the optimizer times out when
it finds a plan whose estimated cost is smaller than the amount of time it took to find the
plan). If the estimated cost had been closer to the actual cost (less than a second), the
optimizer would probably have timed out and returned a plan much earlier.

> Query runs 10 times more slowly with optimizer turned ON
> --------------------------------------------------------
>                 Key: DERBY-4971
>                 URL: https://issues.apache.org/jira/browse/DERBY-4971
>             Project: Derby
>          Issue Type: Bug
>          Components: SQL
>    Affects Versions:
>            Reporter: Chris Wilson
>         Attachments: optimizer-off.sql, optimizer-on.sql, rita.derby.zip
> Derby takes 10 times as long to execute the attached query on the attached database when
the optimizer is allowed to choose its own join order.
> chris@fen-desktop2(tmp)$ cat ij 
> #!/bin/sh
> DERBY_DIR=/home/chris/.m2/repository/org/apache/derby
> TOOLS_JAR=$DERBY_DIR/derbytools/$DERBY_VER/derbytools-$DERBY_VER.jar
> java -cp $DERBY_JAR:$TOOLS_JAR $OPTIONS org.apache.derby.tools.ij "$@"
> chris@fen-desktop2(tmp)$ time ./ij optimizer-on.sql 
> ij version 10.7
> ij> connect 'jdbc:derby:;databaseName=/tmp/rita.derby;user=rita';
> ij> SELECT journey.transport_company AS journey_transport_provider, ...
> 1 row selected
> ij> quit;
> real	0m47.459s
> user	0m47.067s
> sys	0m0.344s
> chris@fen-desktop2(tmp)$ time ./ij optimizer-off.sql 
> ...
> real	0m3.913s
> user	0m4.100s
> sys	0m0.148s

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