Advanced MySQL 8

EXPLAIN extra information

One of the important columns of EXPLAIN is, of course, the EXTRA column. This column contains additional information on how MySQL will support your query.

A quick note: if you want your queries to be faster, take a look in the EXTRA column and try to see messages such as Using Filesort and Using Temporary Table; we will revisit these two messages later.

Let's take a look at the list of the most important messages you'll find in the EXPLAIN view:

• Full scan on NULL key: This message is displayed when MySQL cannot access an index for a subquery.
• Impossible HAVING: The HAVING clause cannot select any records.
• Impossible WHERE: Similar to HAVING because it cannot find records.
• Not exists: As a general rule, MySQL has ...