A typical scenario
Boss: Our new website is crawling! How can it be, we have four state-of-the-art web servers - what's the problem?
You: Well, the web servers are fine - it's the database server that's struggling.
Boss: What? You told me this MySQL thing was fast, that we didn't need Oracle, and now you say it can't cope! How can this be?
You: Well, the web servers are behaving so well that they're pushing through lots of queries, and the database can't manage to process all of them at the same time. It's only one database, and lots of web servers...
Boss: It's too late to buy Oracle now - what are we going to do!?
Big Boss to Boss(in the boss's mind): This project has been a disaster from the beginning - now you want me to delay it while we install a new database, and spend a whole lot more! Do you think we're made of money!? I'm calling in someone who knows what they're doing - you're history buddy.
Colleague (about to take your job): Wait, I think I can solve the problem!

So, what does your colleague know that you don't? How can he save the day and let the boss get all the credit? Our scenario is too imprecise to generalize, and there are many possible solutions. You can read about optimizing queries and indexes, optimizing by improving the hardware, and tweaking the MySQL variables, using the slow query log, and of course, there are other methods such as replication. However, MySQL 4 provides one feature that can prove very handy - a query cache. In a situation where the database has to repeatedly run the same queries on the same data set, returning the same results each time, MySQL can cache the result set, avoiding the overhead of running through the data over and over. Usually, you would want to implement some sort of caching on the web server, but there are times when this is not possible, and then it is the query cache you will look to for help.

Setting up the query cache
To make sure MySQL uses the query cache, there are a few variables you need to set in the configuration file (usually my.cnf or my.ini). First, is the query_cache_type. There are three possible settings: 0 (for off, do not use), 1 (for on, cache queries) and 2 (on demand, discussed more below). To ensure it is always on, place:

query-cache-type = 1
in the configuration file. If you started the server having only made this change, you would see the following cache variables set:

mysql> SHOW VARIABLES LIKE '%query_cache%';
+-------------------+---------+
| Variable_name     | Value   |
+-------------------+---------+
| have_query_cache  | YES     |
| query_cache_limit | 1048576 |
| query_cache_size  | 0       |
| query_cache_type  | ON      |
+-------------------+---------+
4 rows in set (0.06 sec)

Note that these are results from MySQL 4.0.x - you'll see more in versions 4.1.x and beyond. The query_cache_type will be set to ON or OFF as appropriate. However, there is one more to set, and that is the query_cache_size. If set to 0 (the default), the cache will be disabled. This variable determines the memory, in bytes, used for the query cache. For our purposes, we will set it to 20 MB:

query-cache-size = 20M
The amount is shown in bytes:

mysql> SHOW VARIABLES LIKE '%query_cache%';
+-------------------+----------+
| Variable_name     | Value    |
+-------------------+----------+
| have_query_cache  | YES      |
| query_cache_limit | 1048576  |
| query_cache_size  | 20971520 |
| query_cache_type  | ON       |
+-------------------+----------+
4 rows in set (0.06 sec)

The Query cache in action (almost)
For this tutorial, I used a dump from Wikipedia, the open content encycl