The table is so small that it is faster to perform a table scan than 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 ON or WHERE clause 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 Sección 7.2.4, “Optimización de las cláusulas WHERE por parte de MySQL”.

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 it probably does a lot of key lookups and that a table scan would be faster.

Use ANALYZE TABLE tbl_name to update the key distributions for the scanned table. See Sección 13.5.2.1, “Sintaxis de ANALYZE TABLE”.

Use FORCE INDEX for the scanned table to tell MySQL that table scans are very expensive compared to using the given index. See Sección 13.2.7, “Sintaxis de SELECT”.

SELECT * FROM t1, t2 FORCE INDEX (index_for_column)
WHERE t1.col_name=t2.col_name;

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 Sección 5.3.3, “Variables de sistema del servidor”.