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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, it means that you are comparing to a constant value rather than say, another column, but it does not tell you what that value is. The wording in the manual indicates it will tell you the constant’s value, but that is not correct.
The important takeaway here is that you can can see what is being compared to what. For example, a constant value to an index.