ACC2000: Compound Indexes Must Restrict First Indexed Field (209564)

SUMMARY

Before an index can be used, queries with restrictions on multiple-field (compound) indexes must restrict at least the first field of the index. You can, however, also use adjacent fields in the index (up to 10 fields). If the restriction is placed on a field other than the first field, the query optimizer scans the table, rather than using the index. This is not always desirable, because table scans are slower than index searches for most queries.

MORE INFORMATION

By default, when a compound index is created in Microsoft Access, no individual indexes are assigned to the fields included in the compound index. This behavior is by design.

For the query optimizer to use an index, you must use a comparison of either the first field in the compound index or the first field and any number of adjacent fields (up to 10) that make up the compound index. You must query the indexed fields in the order that they appear in the Indexes dialog box, beginning with the first indexed field and continuing with adjacent fields.

NOTE: This principle also applies to using criteria with the Find method in Visual Basic for Applications.

For example, consider a table (T1) that has three fields: key_part1, key_part2, and key_part3. If there is a composite index created on these three fields and all fields are the primary key, then the following SQL statement does not use the index, because the first field, key_part1, is not being used.

SELECT * FROM T1 WHERE key_part2 = <value>

Neither does the following SQL Statement make use of the index. Although key_part1 is referred to, key_part1 and key_part3 are not adjacent fields.

SELECT * FROM T1 WHERE key_part1 = <value> AND key_part3 = <value>

However, each of the following three SQL statements will use the index because they each include the first field, or the first field and one or more adjacent fields of the composite index:

SELECT * FROM T1 WHERE key_part1 = <value>

SELECT * FROM T1 WHERE key_part1 = <value> AND key_part2 = <value>

SELECT * FROM T1 WHERE key_part1 = <value> AND key_part2 = <value> AND key_part3 = <value>

The above fields are not prohibited from having individual indexes on them. Individual indexes can be built for each field, allowing comparisons on those fields with index searches. Be aware, however, that indexes can take up as much (or more) space than the data.

REFERENCES

For more information about creating indexes, click Microsoft Access Help on the Help menu, type creating an index in the Office Assistant or the Answer Wizard, and then click Search to view the topics returned.

For more information about optimizing queries, click Microsoft Access Help on the Help menu, type optimize performance in the Office Assistant or the Answer Wizard, and then click Search to view the topics returned.