MySQL Reference Manual :: 7.2.13 How to Avoid Table Scans

The table is so small that it's faster to do a table scan than a key lookup. This is a common case 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 Section 7.2.4, “How MySQL Optimizes WHERE Clauses”.

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 Section 13.5.2.1, “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. See Section 13.1.7, “SELECT Syntax”.

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 Section 5.2.3, “Server System Variables”.