08.04.2011 18:42, Patrick Nagel пишет:
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> Hi,
>
> I first encountered this in October 2010, when I switched to MySQL 5.1.50 on
> a testing system (while the production system is still running MySQL
> 5.0.91). The simple query
>
> {{#ask: [[Category:Freelancers]] [[Joe Smith]] }}
>
> suddenly returned nothing on the testing system, although page "Joe Smith"
> is in the Freelancers category.
>
> After lots of digging through all the layers, ruling out one component after
> another, I can now say with confidence, that the problem is in MySQL server
> versions 5.1.49, 5.1.50, 5.1.51 and 5.1.52. I have been told that it's
> caused by a bug in the 'index_merge' behaviour, which is present in these
> versions.
>
> For details, have a look at the testcase I built and documented at [1]. A
> simple SQL query like "SELECT * FROM smw_inst2 WHERE smw_inst2.s_id=9877 AND
> smw_inst2.o_id=650;" exposes the bug.
>
> I have been told that SMW could work around that index_merge bug by changing
> the separate indexes to a compound index. I tried it for the smw_inst2
> table, and SMW then gives the expected result in the exact query above.
> There are other non-compound indexes in various SMW database tables though,
> and I don't know enough about the exact problems caused by the index_merge
> bug to be sure that transforming the indexes of just the smw_inst2 table
> into a compound index will guarantee that no other queries will return
> incorrect results. It would be great if someone with deeper knowledge of
> MySQL and indexes could clarify.
>
> For now, I recommend to avoid these MySQL server versions.
>
> Patrick.
>
>
Thanks for reporting! Some conditional check warning on server side (in
PHP code of SMW) would be nice to have.
Dmitriy
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