So, I'm just back from a party and I'm still a little tipsy, but here goes. . .

It's not always about being faster. Sometimes, it's about being more scalable. If I gave you a computer twice as fast as mine and told you to O(n^2) sort a million things, it would take you a lot longer than me using the slower machine, but using an O(n * log n) algorithm.

This gets us into the difference between MySQL and memcached. With MySQL I can say "give me the people where the first name is 'Brian' and the last name is not 'Applestein'" Awesome! Convenient, useful, slow :-(. With memcached, I can say, "give me key 'Per:48'". Um, not that useful or convenient, but fast as hell.

Why? What memcached is doing is creating a hash table. Basically, it knows where something is going to be stored because of its key. So, 'Per:48' will always be in place X because that's where it hashes to. When I want to retrieve that key, it doesn't have to look. It can just go! That means that it runs in constant Order(1) time. MySQL is going to use an index (if you're lucky). That index is going to sort the items in a specific order and then MySQL can retrieve an item in log n time. log n > 1 for almost all values (sorry, I don't deal with math below like, 10 since it's never relevant in my work since, well, scaling to 3 items doesn't matter).

So, even if MySQL kept everything in memory, it wouldn't be as scalable as memcached. memcached is able to do an Order(1) hash lookup. MySQL (with an index scan) is log n. This brings me to the query cache. While I don't pretend to know all of the internals of these technologies, I'm guessing that MySQL's query cache is basically like memcached. You have another key -> value store. Basically, MySQL has stored the result set as the value of the key which is the query.

The problem with the query cache is that it becomes invalid the second something is changed in the database, thus making it useless for scale.

So, why does memcached overcome that problem? Well, part of it is that when you scale, you leave behind those awesome queries that are so neat. You want to get all of the friends of "Matt"? Well, grab the serialized object "Matt" from memcached. That should have a list of the ids of his friends. Ask memcached for the objects matching that list of keys. As you can see, memcached isn't relying on a query of Matt's friends. Maybe you have never done that before. BUT, you have put each record into memcached and therefore it is available in this context. Where the entire query cache becomes invalid everytime someone hits the database, you can hack MySQL to update memcached with the one record that has been updated without invalidating the things that haven't changed.

If you have questions, you can always respond and I'll try to answer in the morning. Basically, querying is never infinitely scalable. It's log n. Hash lookups can scale. While more memory makes a database faster and scales it higher, it doesn't match a cache like memcached because queries are a more costly operation than hash lookups. Yeah, it's a pain. You and I probably won't have to deal with it much since databases are pretty damn efficient and log n is pretty good. If you become the next Twitter, hire someone who can explain this comment. Before then, don't worry so much! You'll give yourself wrinkles!