MySQL 9.1 Release Notes
Depending on the details of your tables, columns, indexes, and the
conditions in your WHERE clause, the MySQL
optimizer considers many techniques to efficiently perform the
lookups involved in an SQL query. A query on a huge table can be
performed without reading all the rows; a join involving several
tables can be performed without comparing every combination of
rows. The set of operations that the optimizer chooses to perform
the most efficient query is called the “query execution
plan”, also known as the
EXPLAIN plan. Your goals are to
recognize the aspects of the
EXPLAIN plan that indicate a query
is optimized well, and to learn the SQL syntax and indexing
techniques to improve the plan if you see some inefficient
operations.