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B.4.5 Optimizer-Related Issues
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 does not 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:EXPLAINis discussed in more detail in Section 13.8.2, “EXPLAIN Syntax”.Use
ANALYZE TABLEto update the key distributions for the scanned table. See Section 13.7.3.1, “ANALYZE TABLE Syntax”.tbl_nameUse
FORCE INDEXfor the scanned table to tell MySQL that table scans are very expensive compared to using the given index:USE INDEXandIGNORE INDEXmay also be useful. See Section 8.9.4, “Index Hints”.Global and table-level
STRAIGHT_JOIN. See Section 13.2.10, “SELECT Syntax”.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 5.1.8, “Server System Variables”.
Document created the 26/06/2006, last modified the 26/10/2018
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