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:
Use the EXPLAIN
statement to
get information about how MySQL processes a query. To use
it, just add the keyword
EXPLAIN
to the front of your
SELECT
statement:
mysql> EXPLAIN SELECT * FROM t1, t2 WHERE t1.i = t2.i;
EXPLAIN
is discussed in more
detail in Section 12.8.2, “EXPLAIN
Syntax”.
Use ANALYZE TABLE
to update the
key distributions for the scanned table. See
Section 12.4.2.1, “tbl_name
ANALYZE TABLE
Syntax”.
Use FORCE INDEX
for the scanned table to
tell MySQL that table scans are very expensive compared to
using the given index:
SELECT * FROM t1, t2 FORCE INDEX (index_for_column) WHERE t1.col_name=t2.col_name;
USE INDEX
and IGNORE
INDEX
may also be useful. See
Section 12.2.9.2, “Index Hint Syntax”.
Global and table-level STRAIGHT_JOIN
. See
Section 12.2.9, “SELECT
Syntax”.
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
Section 5.1.4, “Server System Variables”.
MySQL Enterprise. For expert advice on configuring MySQL servers for optimal performance, subscribe to the MySQL Enterprise Monitor. For more information, see http://www.mysql.com/products/enterprise/advisors.html.
User Comments
Add your own comment.