For a query with an ORDER BY or GROUP BY and a LIMIT clause, the optimizer tries to choose an ordered index by default when it appears doing so would speed up query execution. Prior to MySQL 5.7.33, there was no
❝If multiple rows have identical values in theORDER BYcolumns, the server is free to return those rows in any order, and may do so differently depending on the overall execution plan. In other words, the sort order of those rows is nondeterministic with respect to the nonordered columns....
This section describes when MySQL can use an index to satisfy an ORDER BY clause, the filesort operation used when an index cannot be used, and execution plan information available from the optimizer about ORDER BY. An ORDER BY with and without LIMIT may return rows in different orders, as...
在5.6中的执行步骤与5.7基本一致,只不过converting HEAP to ondisk变成了converting HEAP to MyISAM,MySQL 5.6的默认临时表是myisam表,这里为什么会比5.7的速度快有待研究: {"join_execution":{"select#":1,"steps":[{"creating_tmp_table":{// 创建临时表"tmp_table_info":{"table":"intermediate_tmp_tab...
If multiple rows have identical values in the ORDER BY columns, the server is free to return those rows in any order, and may do so differently depending on the overall execution plan. In other words, the sort order of those rows is nondeterministic with respect to the nonordered columns....
For a query with an ORDER BY or GROUP BY and a LIMIT clause, the optimizer tries to choose an ordered index by default when it appears doing so would speed up query execution. Prior to MySQL 5.7.33, there was no way to override this behavior, even in cases where using some other opt...
SQL Lesson 12: Order of execution of a Query Now that we have an idea of all the parts of a query, we can now talk about how they all fit together in the context of a complete query. Complete SELECT query SELECT DISTINCT column, AGG_FUNC(column_or_expression), … FROM mytable JOIN...
http://stackoverflow.com/questions/24127932/mysql-query-clause-execution-order The actual execution of SQL statements is a bit tricky. However, the standard does specify the order of interpretation of elements in the query. This is basically in the order that you specify, although I thinkhaving...
/* 计算 Innodb_rows_read 的差值 */mysql>select@b-@a; 查看OPTIMIZER_TRACE结果中的number_of_tmp_files字段确认是否使用临时文件。 "filesort_execution": [],"filesort_summary": {"rows":4000"examined_rows":4000,"number_of_tmp_files":12,...
Make sure it uses index It is very important to have ORDER BY with LIMIT executed without scanning and sorting full result set, so it is important for it to use index – in this case index range scan will be started and queryexecution stopped as soon as soon as required amount of rows...