Changing Table Structures - Mastering phpMyAdmin 3.1 for Effective MySQL Management - page 109

We can see that the EXPLAIN command has been passed to MySQL, telling us that
the possible_keys used is author_language. Thus, we know that this index will be
used for this type of query. If this index had not existed, the result would have been
quite different.
Here, possible_keys ( NULL ) and the type ( ALL ) mean that no index would be used,
and all rows would need to be examined to find the desired data. Depending on
the total number of rows, this could have a serious impact on the performance. We
can ascertain the exact impact by examining the query timing that phpMyAdmin
displays on each result page ( Query took x sec ), and comparing it with or without
the index. However, the difference in time can be minimal if we only have limited
test data, compared to a real table in production.
Detection of index problems
To help users maintain an optimal index strategy, phpMyAdmin tries to detect some
common index problems. For example, let's access the book table and add an index
on the isbn column. When we display this table's structure, we get a warning:
The intention here is to warn us about an inefficient index structure when
considering the whole table. We don't need to have two indexes on the same column.