New Features of MySQL 5.0 database Stored Procedures

When you submit a query, MySQL will analyze it to see if some optimization can be done to make it faster to process the query. This section describes how the query optimizer works. If you want to know the optimization methods used by MySQL, you can refer to the MySQL reference manual.

Of course, the MySQL query optimizer also uses indexes, but it also uses other information. For example, if you submit a query as shown below, MySQL runs the query very quickly regardless of the data table size:

SELECT * FROM tbl_name WHERE 0;

In this example, MySQL checks the WHERE clause and recognizes that there are no data rows that meet the query conditions. Therefore, it does not consider searching data tables. You can see this situation by providing an EXPLAIN statement, which allows MySQL to display some information about the SELECT query that has been executed but has not actually been executed. If you want to use EXPLAIN, you only need to put the EXPLAIN word before the SELECT statement:

mysql> EXPLAIN SELECT * FROM tbl_name WHERE 0\G *************************** 1. row *************************** id: 1 select_type: SIMPLE table: NULL type: NULL possible_keys: NULL key: NULL key_len: NULL ref: NULL rows: NULL Extra: Impossible WHERE

In general, EXPLAIN returns more information than the above information, it also includes non-NULL information such as the index used to scan the data table, the join type used, and the estimated number of data rows in each data table to be checked.

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