MySQL uses a cost-based optimizer to determine the best way to resolve a query. In many cases, MySQL can calculate the best possible query plan, but sometimes MySQL doesn't have enough information about the data at hand and has to make 「educated」 guesses about the data.
For the cases when MySQL does not do the "right" thing, tools that you have available to help MySQL are:
EXPLAIN statement to get
information about how MySQL processes a query. To use it,
just add the keyword
EXPLAIN to the front
EXPLAIN SELECT * FROM t1, t2 WHERE t1.i = t2.i;
EXPLAIN is discussed in more detail in
to update the
key distributions for the scanned table. See
ANALYZE TABLE 構文」.
FORCE INDEX for the scanned table to
tell MySQL that table scans are very expensive compared to
using the given index. See 「
SELECT * FROM t1, t2 FORCE INDEX (index_for_column) WHERE t1.col_name=t2.col_name;
USE INDEX and
INDEX may also be useful.
Global and table-level
You can tune global or thread-specific system variables. For
example, Start mysqld with the
--max-seeks-for-key=1000 option or use
SET max_seeks_for_key=1000 to tell the
optimizer to assume that no key scan causes more than 1,000
key seeks. See 「システム変数」.