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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.
[1] http://p173.de/gp/index.php?id=7e1bee97ab&view=nl
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