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7.2.11.2. Tight index scan

A tight index scan may be either a full index scan or a range index scan, depending on the query conditions.

When the conditions for a loose index scan are not met, it is still possible to avoid creation of temporary tables for GROUP BY queries. If there are range conditions in the WHERE clause, this method reads only the keys that satisfy these conditions. Otherwise, it performs an index scan. Since this method reads all keys in each range defined by the WHERE clause, or scans the whole index if there are no range conditions, we term it a tight index scan . Notice that with a tight index scan, the grouping operation is performed only after all keys that satisfy the range conditions have been found.

For this method to work, it is sufficient that, for all columns in a query referring to parts of the key coming before or in between parts of the GROUP BY key, there is a constant equality condition. The constants from the equality conditions fill in any “gaps” in the search keys so that it is possible to form complete prefixes of the index. These index prefixes can be then used for index lookups. If we require sorting of the GROUP BY result, and it is possible to form search keys that are prefixes of the index, MySQL also avoids extra sorting operations because searching with prefixes in an ordered index already retrieves all the keys in order.

The following queries do not work with the first method above, but still work with the second index access method (assuming we have the aforementioned index idx on table t1):

  • There is a gap in the GROUP BY, but it is covered by the condition c2 = 'a'. 

    SELECT c1, c2, c3 FROM t1 WHERE c2 = 'a' GROUP BY c1, c3;

  • The GROUP BY does not begin with the first part of the key, but there is a condition that provides a constant for that part: 

    SELECT c1, c2, c3 FROM t1 WHERE c1 = 'a' GROUP BY c2, c3;

Ésta es una traducción del manual de referencia de MySQL, que puede encontrarse en dev.mysql.com. El manual de referencia original de MySQL está escrito en inglés, y esta traducción no necesariamente está tan actualizada como la versión original. Para cualquier sugerencia sobre la traducción y para señalar errores de cualquier tipo, no dude en dirigirse a mysql-es@vespito.com.