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https://issues.apache.org/jira/browse/DERBY-4971?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12981763#action_12981763
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Knut Anders Hatlen commented on DERBY-4971:
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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: 10.7.1.1
> 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
> DERBY_VER=10.7.1.1
> DERBY_JAR=$DERBY_DIR/derby/$DERBY_VER/derby-$DERBY_VER.jar
> TOOLS_JAR=$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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