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optimizer_switch flags for next 5.3 release



Written down notes from discussion about which flags should be enabled by
default in the next 5.3 release:

- Derived tables and keys in them should be enabled
- BKA should not be enabled (doesn't make sense for 'default' query loads)
- semi-joins should be enabled 
- Materialization (base) should be enabled
- extra strategies for NULL-aware materialization - ???
- ICP should be enabled
- Hash join could be enabled but in the next minor release.

- Cost-based MRR should not be enabled as cost model is not tuned.

when some feature is considered stable enough, its owner should apply for
testing with {$current_optimizer_switch_default+new_feature}, and once that is
passed, change the default in the main branch.

Sergei Petrunia, Software Developer
Monty Program AB, http://askmonty.org
Blog: http://s.petrunia.net/blog

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