Query Cache support

[Seppo Jaakola]

Query Cache has been declared as non-supported feature. The problem has been that high priority transactions (slave applier) could not get lock free access to query cache resources when invalidating query cache blocks or when aborting a victim possessing locks in query cache.

[Seppo Jaakola (seppo-jaakola)]

Tested Query Cache support in wsrep-5.6, and observed one unresolved deadlock when BF thread could not signal a victim, which was hanging in query cache end state. Fixed this deadlock and enabled query cache options in wsrep-5.6 revision:
http://bazaar.launchpad.net/~codership/codership-mysql/5.6/revision/4007

[Teemu Ollakka (teemu-ollakka)]

Re-enabled start time query cache option checks for 5.6 in http://bazaar.launchpad.net/~codership/codership-mysql/5.6/revision/4019

Backported r4007, r4019 from 5.6 to 5.5: http://bazaar.launchpad.net/~codership/codership-mysql/wsrep-5.5/revision/3928

[Teemu Ollakka (teemu-ollakka)]

Query cache cannot still be fully enabled during startup. To enable query cache, mysqld should be started with query_cache_type=1 and query_cache_size=0 and then query_cache_size should be changed to desired value during runtime.

[Raghavendra D Prabhu (raghavendra-prabhu)]

Moved the query cache support to next milestone since evaluation wrt. query cache changes of Percona Server 5.5 (specifically mutex - http://www.percona.com/doc/percona-server/5.5/performance/query_cache_enhance.html is required). PS 5.6 doesn't have that differently from upstream, hence that is ok.

[Shahriyar Rzayev (rzayev-sehriyar)]

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PXC-1507