This page describes the enhancements for the query cache. At the moment three features are available:
- Disabling the cache completely
- Diagnosing contention more easily
- Ignoring comments
This features provides a new thread state - Waiting on query cache mutex. It has always been difficult to spot query cache bottlenecks because these bottlenecks usually happen intermittently and are not directly reported by the server. This new thread state appear in the output of SHOW PROCESSLIST, easing diagnostics.
Imagine that we run three queries simultaneously (each one in a separate thread):
> SELECT number from t where id > 0; > SELECT number from t where id > 0; > SELECT number from t where id > 0;
If we experience query cache contention, the output of SHOW PROCESSLIT will look like this:
> SHOW PROCESSLIST; Id User Host db Command Time State Info 2 root localhost test Sleep 2 NULL 3 root localhost test Query 2 Waiting on query cache mutex SELECT number from t where id > 0; 4 root localhost test Query 1 Waiting on query cache mutex SELECT number from t where id > 0; 5 root localhost test Query 0 NULL
This feature adds an option to make the server ignore comments when checking for a query cache hit. For example, consider these two queries:
/* first query */ select name from users where users.name like 'Bob%'; /* retry search */ select name from users where users.name like 'Bob%';
By default (option off), the queries are considered different, so the server will execute them both and cache them both.
If the option is enabled, the queries are considered identical, so the server will execute and cache the first one and will serve the second one directly from the query cache.