8.3.1 How MySQL Uses Indexes

To find the rows matching a WHERE clause quickly.

To eliminate rows from consideration. If there is a choice between multiple indexes, MySQL normally uses the index that finds the smallest number of rows (the most selective index).

If the table has a multiple-column index, any leftmost prefix of the index can be used by the optimizer to look up rows. For example, if you have a three-column index on (col1, col2, col3), you have indexed search capabilities on (col1), (col1, col2), and (col1, col2, col3). For more information, see Section 8.3.6, “Multiple-Column Indexes”.

To retrieve rows from other tables when performing joins. MySQL can use indexes on columns more efficiently if they are declared as the same type and size. In this context, VARCHAR and CHAR are considered the same if they are declared as the same size. For example, VARCHAR(10) and CHAR(10) are the same size, but VARCHAR(10) and CHAR(15) are not.

For comparisons between nonbinary string columns, both columns should use the same character set. For example, comparing a utf8 column with a latin1 column precludes use of an index.

Comparison of dissimilar columns (comparing a string column to a temporal or numeric column, for example) may prevent use of indexes if values cannot be compared directly without conversion. For a given value such as 1 in the numeric column, it might compare equal to any number of values in the string column such as '1', ' 1', '00001', or '01.e1'. This rules out use of any indexes for the string column.

To find the MIN() or MAX() value for a specific indexed column key_col. This is optimized by a preprocessor that checks whether you are using WHERE key_part_N = constant on all key parts that occur before key_col in the index. In this case, MySQL does a single key lookup for each MIN() or MAX() expression and replaces it with a constant. If all expressions are replaced with constants, the query returns at once. For example:

To sort or group a table if the sorting or grouping is done on a leftmost prefix of a usable index (for example, ORDER BY key_part1, key_part2). If all key parts are followed by DESC, the key is read in reverse order. (Or, if the index is a descending index, the key is read in forward order.) See Section 8.2.1.15, “ORDER BY Optimization”, Section 8.2.1.16, “GROUP BY Optimization”, and Section 8.3.13, “Descending Indexes”.

In some cases, a query can be optimized to retrieve values without consulting the data rows. (An index that provides all the necessary results for a query is called a covering index.) If a query uses from a table only columns that are included in some index, the selected values can be retrieved from the index tree for greater speed: