The filtered column shows an estimated percentage of rows that will be filtered by the table WHERE clause. It’s typically only shown during the EXPLAIN EXTENDED command.
The “ref” column shows which columns or constants are compared to the index named in the “key” column to select rows from the table. This comes right from the manual, but I have noticed that when the value is const,
These relate to the usage of indexes: possible_keys Which indexes were considered? key Which indexes did the optimizer choose? key_len How many bytes of the index will be used?
The “type” column indicates how MySQL will access rows From worse to better: ALL index range ref eq_ref const system NULL
I recently had to rewrite a query that was running very slowly. It was taking over 30 seconds. Upon analyzing it, I saw that it had 2 correlated sub-queries in it, and each was doing a count(*). This meant that