Updated Query Cache documentation

git-svn-id: svn+ssh://kelev.kaltura.com/usr/local/kalsource/backend/server/trunk/core@62953 6b8eccd3-e8c5-4e7d-8186-e12b5326b719

doc/Query Cache Overview.htm

@@ -16,9 +16,10 @@ <h3>How does it work ?</h3>
 Let's take an example: <pre>Select * from flavor_asset where entry_id=x and partner_id=y and …</pre>
 The first time this query is performed, it will get to the database. After the query completes and the
 Propel objects are populated, the objects are serialized and stored to the memcache. The key used 
-for caching is a hash of the query string (actually, we use md5(serialize($criteria)) to avoid building the 
-query string unnecessarily). The next time the same query is performed it will return the results from
-the memcache.
+for caching is a function of the query string: QCQ-&lt;query type&gt;-&lt;md5(serialize($criteria))&gt;,
+where query type is 'sel' for doSelect and 'cnt' for doCount (we use serialize($criteria) to avoid 
+building the query string unnecessarily). 
+The next time the same query is performed it will return the results from the memcache. 
 </p>
 <h3>What is the performance gain ?</h3>
 <p>
@@ -32,7 +33,7 @@ <h3>How do we know when a cached query is valid ?</h3>
 <p>
 Every query that is cached is associated with at least one 'invalidation key', each invalidation key holds
 the time of the last relevant change to the database, in the example above, we use the key:
-'flavor_asset:entry_id=x'. Before we return a cached query from the memcache, we compare the time
+'QCI-flavor_asset:entry_id=x'. Before we return a cached query from the memcache, we compare the time
 of the cached query to the time saved in all relevant invalidation keys. If one of the invalidation keys
 is newer than the cached query, the cached query is treated as invalid and won't be used.
 </p>
@@ -80,6 +81,7 @@ <h4>Multi datacenter environment only</h4>
 <li>Configure the 'Memcached Functions for MySQL' library to use the shared memcache server by adding the
 command 'select memc_servers_set('&lt;global memcache host&gt;:&lt;global memcache port&gt;');' to the mysql init script.<br>
 <b>Note:</b> To add an init script for mysql, add the command line switch '--init-file=&lt;mysql init script path&gt;' to mysqld.
+The switch should be added to the line starting with /usr/bin/mysqld_safe in /etc/init.d/mysqld.
 <li>Restart mysql.
 <li>Install the triggers by running from deployment/base/scripts: php createQueryCacheTriggers.php create &lt;host&gt; &lt;user&gt; &lt;password&gt;
 </ul>