![]() ![]() Small tables or queries that select a large portion of a table, it can be faster for MySQL to skip the ![]() Quickest way to get the data it needs, which sometimes means choosing a slightly counter-intuitive method. Paradoxically, an index is only sometimes the fastest way to access data! The MySQL optimizer will always try to pick Usually, you won't need to worry about the table sampling or updating the statistics manually. This is beyond the scope of this article, but the MySQL documentation has a comprehensive page on the topic. If you find that the statistics need to be more accurate for a particular table and are continually causing the optimizer to make poor decisions, you can change how the sampling is done. Problems, you can force an update by running ANALYZE TABLE. If the statistics are so outdated that it is causing This happens in the background, and you shouldn't ever notice it. The Cardinality column shows you the stored value MySQL will use to make its selectivity decisions. | people | 1 | state | state | A | 1 | BTREE | YES | | people | 1 | first_name | first_name | A | 3028 | BTREE | YES | | people | 0 | PRIMARY | id | A | 491583 | BTREE | YES | | Table | Non_unique | Key_name | Column_name | Collation | Cardinality | Index_type | Visible | You can see this stored value by running SHOW INDEXES from : It would be inefficient for MySQL to calculate that each time, so instead, it keeps track of the cardinality over time using random sampling. We've been calculating cardinality by running COUNT(DISTINCT column) each time. ![]() To speed this choice up, MySQL keeps statistics about the data's rough shape, which can become outdated. So while checking average selectivity is a good rule of thumb, pay careful attention to unevenly distributed data.Īll other things being equal, MySQL will choose the most selective index possible. But when you're querying for admins, it is highly selective. In this case, an index on type might seem useless because it's not very selective on average. In some cases,Īn index might be highly selective for one query and not selective at all for another.Ĭonsider a table of one million users, where 99% are of type = "user" and 1% are of type = "admin". In the case where multiple indexes might work, the optimizer must make a decision between multiple viable options.Ĭonsider the following query, which searches for people named "Aaron" that live in Texas:Ĭalculating selectivity across an entire table can be misleading if the data is not evenly distributed. In our example above, only one possible index could satisfy the query, so the optimizer doesn't have to choose which one isīest. Now that we know how to determine what index is (or isn't!) being used, let's look at some of the reasons why your index Having decided on the indexes that apply to the query, it must then choose between those indexes as to which MySQL's query optimizer, responsible forĭetermining the best way to execute a query, looks at the query and the available indexes and decides which indexes areĪpplicable. These are separate pieces of information, both of whichĪre valuable! Before your index can be chosen, it must first be considered. Our index on first_name was considered, and it was chosen. From installing a text editor to understanding how a web browser interacts with a web server to handling events with JQuery, you’ll gain a complete introductory overview of web application development.Check out this article for more a more in-depth guide on how to read an EXPLAIN You’ll demonstrate basic concepts, like database design, while working on assignments that require the development of increasing challenging web apps. This Specialization (and its prerequisites) will prepare you, even if you have little to no experience in programming or technology, for entry level web developer jobs in PHP. Over the course of this Specialization, you will create several web apps to add to your developer portfolio. You will develop web and database applications in PHP, using SQL for database creation, as well as functionality in JavaScript, jQuery, and JSON. Web Applications for Everybody is your introduction to web application development. This Specialization is an introduction to building web applications for anybody who already has a basic understanding of responsive web design with JavaScript, HTML, and CSS. ![]()
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