3/3/2023 0 Comments Mysql optimizerYou can retrieve the results of the trace from the INFORMATION_SCHEMA.OPTIMIZER_TRACE table. Then you run the query you want to trace. Enable MySQL optimizer tracingīe warned though, the output can be long. This is achieved by setting optimizer_trace="enabled=on". To trace the MySQL optimizer, you have to turn it on for your session. Luckily, you have the ability to trace the MySQL optimizer to find out why. Unfortunately, EXPLAIN won’t give you the why, only the how. When selecting for that user, MySQL could decide it costs less to do a full table scan than using that nice user_id index you have on the table. It’s early stage of your application, so you have one user that contains 90% of your activity. Imagine if you have a table that tracks all of your user activity. Because the values change the selectivity. It’s also common that the path can change based on the values of the query. Some times, it is because the optimizer chose a different path. This has told you how MySQL has resolved the query.īut have you ever looked at a query that suddenly started performing poorly, and been left wondering why? Well, if you have ever tried investigating the performance of a query, I assume you have looked at the EXPLAIN output. The optimizer determines the cost to retrieve results. It’s a bit of light reading that requires at least 2 cups of coffee! Why trace the optimizer If you are interested, the MySQL documentation shows the different primary optimizations, as well as other optimizations. In really simple terms, it estimates a cost for each potential path to get the query results based on how the query is constructed. To keep query performance fast, MySQL has to make a quick decision of which path to take. The number of ways to get the results gets even more complicated if the query contains subqueries or joins. There are different types of scans across the rows that can be used which depends on the filters in the WHERE clause. Simple queries can either be a full table scan, or utilize an index. So, query performance is critical for any serious application.Ī database like MySQL will likely have multiple ways to retrieve the results of a query. It is unlikely that you want to make your users wait more than a few milliseconds to load that data. A typical application will make many calls to the database to gather information for the end user.
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