Derived2 mysql explain

Where foo is a derived table. In your case they are not so explicit. This is probably due to the fact that you are querying against views with views in them, and lord only knows how deep it goes. Each subquery is within a box that represent a temporary table. The Visual EXPLAIN diagram shows that the temporary table for the derived table twill be scanne and for each row, an index lookup into the temporary table for twill be made.

The optimizer determines information about derived tables in such a way that EXPLAIN does not need to materialize them. Optimizing Derived Tables”. It is possible under certain circumstances that using EXPLAIN SELECT will modify table data. This can occur if the outer query accesses any tables and an inner query invokes a stored function that changes one or more rows of a table. EXPLAIN is one of the most powerful tools at your disposal for understanding and optimizing troublesome MySQL queries , but it’s a sad fact that many developers rarely make use of it.

Your sub-query is the issue. Speaking about Views with group by – good question. This is a MySQL restriction that is lifted in MySQL 8. For example, the derived table dt in the following query contains a reference t1. Introduction to MySQL Common Table Expression (CTE), an alternative to a derived table.

It simplifies complex joins and subqueries, improving the readability of the queries. I have omitted an interesting new feature in latest MySQL release 8. I covered last month. The mysql command line from where I was making this request was idle and doing nothing.

The observed effects are manifestations of the fact that at the moment FROM subqueries are resolved using this strategy: 1. A segunda parte da tua duvida, podemos ler aqui (Inglês): The rows column indicates the number of rows MySQL believes it must examine to execute the query. MySQL Workbench의 Visual EXPLAIN 기능에 의해 생성된 것이다. We make web hosting easy, but do our best to explain it in plain English too. The EXPLAIN output shows that it has to go through 14048rows, which is about all the rows in the table. The Extra column indicates that MySQL tries to reduce the amount of rows it inspects using the WHERE clause, but it estimates to reduce those to , which is.

Subqueries in the FROM clause are executed even for the EXPLAIN statement (that is, derived temporary tables are materialized). This occurs because upper-level queries need information about all tables during the optimization phase, and the table represented by a subquery in the FROM clause is unavailable unless the subquery is executed. The EXPLAIN keyword is used throughout various SQL databases and provides information about how your SQL database executes a query. This is because MySQL is unable to determine the types of the columns for the derived table without executing it.

So the query is executed as part of the pre-execution phase. If you continue browsing the site, you agree to the use of cookies on this website. EXPLAIN PARTITIONS is useful for examining queries involving partitioned tables. With the help of EXPLAIN , you can see where you should add indexes to tables so that the statement executes faster by using indexes to find rows.

Obtaining Information About Partitions”.