Bug #21473 why don't the FEDERATED tables work with query cache ?

• Submitted: 7 Aug 2006 7:30
• Modified: 28 Aug 2006 12:38
• Reporter: ming lu
• Status: Won't fix
• Category: MySQL Server: Federated storage engine
• Severity: S4 (Feature request)
• Version: 5.1.11
• OS: Any (all)
• CPU Architecture: Any

[7 Aug 2006 7:30] ming lu

Description:
The mysql5.1 reference manual says that the FEDERATED tables do not work with the query cache, aren't they?  why don't  the FEDERATED tables  work with query cache ? 
I think the query performance against to the FEDERATED tables will be bad.

How to repeat:
Do a test about query performance between federated engine and other normal engine like innodb, you will find the query against to the federated engine tables is slow than others obviously. 
Of course, the remote query must be slow than local query, but i think the query cache is also one of the reasons.

[28 Aug 2006 12:38] Valeriy Kravchuk

There is no efficient way to check if results in query cache are still valid for FEDERATED table (think about it!). So, if FEDERATED tables will be cached you'll get incorrect results, or even worse performance.

Bug #19054 explain some of the reasons why FEDERATED tables are slow.