it's a memory in client side with query_parameters (sql_mode, query text, schema used, and others flags that change results) + query_resultsexample in php
$query_cache= array(
"sql_mode=12341234124124;SELECT * from query_Table" => array(
"tables_used"=>array("query_Table"),
0=>array('a'=>1,'b'=>2,'c'=>3),
2=>array('a'=>1,'b'=>2,'c'=>3),
3=>array('a'=>1,'b'=>2,'c'=>3),
4=>array('a'=>1,'b'=>2,'c'=>3),
)
);
$query_tables_checksum=array("query_Table"=>'some_checksum');
the query_table_checksum is the part of query cache to invalidade a query at client side, it must be checked at each query execution (this can be done in less time or near time of a cached query)
before get from client side cache, query the server about table changes, if the counter (or a checksum?)
if different, the query is invalid and must be removed from client side and reexecuted, if not just get from query cache (in client side)
this add two news function at mysql protocol (one to get table counters/checksums), and maybe must add a table counter that changes after each update/delete/alter and save it (probably) at .FRM file, the other to return the checksum at each query (with this we can see tables used in a update for example and invalid the cache)