The table is so small that it is faster to perform a table scan than to bother with a key lookup. This is common for tables with fewer than 10 rows and a short row length.
There are no usable restrictions in the
WHEREclause for indexed columns.
You are comparing indexed columns with constant values and MySQL has calculated (based on the index tree) that the constants cover too large a part of the table and that a table scan would be faster. See Section 22.214.171.124, “WHERE Clause Optimization”.
You are using a key with low cardinality (many rows match the key value) through another column. In this case, MySQL assumes that by using the key probably requires many key lookups and that a table scan would be faster.
For small tables, a table scan often is appropriate and the performance impact is negligible. For large tables, try the following techniques to avoid having the optimizer incorrectly choose a table scan:
ANALYZE TABLEto update the key distributions for the scanned table. See Section 126.96.36.199, “ANALYZE TABLE Statement”.
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;
Start mysqld with the
--max-seeks-for-key=1000option or use
SET 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”.