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
EXPLAINstatement to get information about how MySQL processes a query. To use it, just add the keywordEXPLAINto the front of yourSELECTstatement:mysql>
EXPLAIN SELECT * FROM t1, t2 WHERE t1.i = t2.i;EXPLAINis discussed in more detail in Section 12.8.2, “EXPLAINSyntax”.Use
ANALYZE TABLEto update the key distributions for the scanned table. See Section 12.4.2.1, “tbl_nameANALYZE TABLESyntax”.Use
FORCE INDEXfor 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 INDEXandIGNORE INDEXmay also be useful. See Section 12.2.9.2, “Index Hint Syntax”.Global and table-level
STRAIGHT_JOIN. See Section 12.2.9, “SELECTSyntax”.You can tune global or thread-specific system variables. For example, start mysqld with the
--max-seeks-for-key=1000option or useSET max_seeks_for_key=1000to tell the optimizer to assume that no key scan causes more than 1,000 key seeks. See Section 5.1.4, “Server System Variables”.