Another part of the MySQL server that can be very important to us are some of the performance caches. These are caches that are created so that certain things can be held in RAM for reuse that gives us better performance than storing those same things on disk or seeking out that information on disk. So what are some of these things? Well one of them is the Query Cache. We saw an example of using the query cache and a really brief explanation of it. This is going to be a little bit more detailed explanation of it, earlier in the chapter. So the query cache is used to speed up data access but it really only works well in read only parts of our database. But on the other hand if you look back at the history of MySQL and what it was originally designed for, it was intended for creating data driven websites where we have a lot of content that we want to feed up to people in essentially a read only fashion. So it makes sense that something like the query cache would have appeared here. The idea behind it is that if a query has already been issued and we've already cached that then we should be able to find that exact query again and send that same information back to the new client that's asking for that. So if something happens to a specific celebrity in a certain week for example, Google would have a query cache out there and people querying for the name of this person would be able to find the same things in the system that the last person did that was looking for that. Because it's been cached and it would come back much faster than if it had to go and search through all of it's vast storage media in order to find the same information. However, it's not much use for tables where the data changes because if a query was cached and then we make a change to that table then that query may not be valid anymore. So if we do an insert, update or delete on the table we really need to get rid of all the queries that were cached for that table because they could have been compromised. So tables that are going to be updated fairly frequently this just isn't going to work for it. But it works really well for tables that are relatively static. So there are many ways that we can manage this and we're going to learn about that but we're not going to learn about this particular thing until we get into some of the optimization items in the Part 2 course. But I want you to at least be aware of the fact that the query cache does exist. Okay. There's another cache called the thread cache and we'll look at this also in Part 2. The thread cache is used for high volume, low persistence access to the server. So what does that mean? Well that would be again data driven websites that are communicated with through PHP in a non persistent manner or an intermittent connection manner. So if we're constantly connecting to the server and disconnecting almost immediately after doing a couple of commands, that could slow down the system if we have hundreds or thousands of people doing that. Because each time we try to connect to the server, a thread has to be created. A thread is a memory object and it starts at 192K but it can grow eventually. But it still has to be created and allocated to this connection and that takes some time. What if instead when somebody were done with that, that we simply strip off all the extra stuff that the thread has accumulated, take it back to it's pristine state and put it into a little cache. So that the next connection that comes along and attempts to connect to the server could just grab this thread rather than having to create a new thread. In a high volume situation that can actually save us a significant amount of time and can make the server appear to be running faster. Because it is running more efficiently at that point. So the idea is that it requires less time to simply grab the thread that has been put into the cache than to create a new thread. So that can be a beneficial thing for us as well and that's why I put this into the category of performance caches. It's not useful for high persistence systems though. So if we're going to connect to the server and hang in there for a while rather than simply perform a couple of lines of code and then log off again. Then the fact that we have a thread cache is going to be insignificant based upon the time that we're planning on spending in the first place. Whereas, if we're constantly logging on and logging off and logging on and logging off and we've got hundreds of people or thousands of people doing that, then we would need a cache like this. So we'll look at how we can decide what would be a good size for this cache. Whether we want to have it switched on at all. There are a number of different decision points but that's going to be in Part 2. And finally for this lesson another cache that we might have is the MyISAM Storage Engine Index Page Cache. MyISAM stores it's indexes in a text file on disk. Now, we wouldn't be able to read that text if we just looked at it but it's still stored as a basic file. The thing is it takes a while to read that. So once we have loaded a page of index information into RAM we can read it much faster and for those situations where we're reading the same rows over and over again, then this can be very useful. If we have a really large table but we're only reading a small number of things for example maybe we have a, a sales or a registration database or something where only the most recent ones are really of interest to us. And the other ones we still have there for certain demographics and historical purpose and so on. But we're really only doing ongoing lookups and ongoing updates and so on with a fairly small number of them toward the bottom end of the file, then caching those indexing pages can be a really good thing. Or if it's a smaller table but we're accessing it a lot, it can still be a good thing, you just have all of the index pages in the cache. So that is another kind of a cache that we can have. I don't plan on doing anything specific with this one right away but you can now that you know that this exists and we will point out some of the variables that manage it. You'll be able to figure out how to use that after the exercises we do for the other ones.
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