REQUEST: I'm not exactly sure what to do with
@@debug_optimizer_prefer_join_prefix, we both should think about how to
use it
(take a query, and try forcing all possible join orders?). At least, it
should
be easier to repeat the bugs now.
Elena,
The reason I asked for this feature is because the RQG detected some
transient semijoin issues that were not repeatable outside of the RQG. In
particular, the RQG reported seeing EXPLAIN plans where "End Temporary" was
before "Start Temporary", which should never happen. Sergey P reported that
it is difficult to build an assertion for this situation that will make the
server crash, so the only option is to find a repeatable MTR test case.
However, STRAIGHT JOIN and FORCE INDEX are not enough to force a particular
ordering of tables where semijoin is involved, as semijoin may place the
tables from the subquery anywhere in the join order of the outer query.
Hence the idea to force an order using a separate variable.
The way to use this feature is to create a new Validator for the RQG that
will
1. Run explain to obtain the names of participating tables
2. Use Bernt's code from combinations.pl --run-all-combinations-once to
generate each possible join order, or at least move around just the
semijoined tables;
3. Let the RQG execute the modified query and compare the result to the
original join order
This can be accomplished with a short Perl module, which would be a nice
exercise in RQG internals. Let me know if you want to do it and I will help
sort out the technicalities.
Philip Stoev
BR
Sergei
--
Sergei Petrunia, Software Developer
Monty Program AB, http://askmonty.org
Blog: http://s.petrunia.net/blog
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