An index helps Microsoft Access find and sort records faster. Microsoft Access uses indexes in a table as you use an index in a book: to find data, it looks up the location of the data in the index. You can create indexes based on a single field or on multiple fields. Multiple-field indexes enable you to distinguish between records in which the first field may have the same value.
You'll probably want to index fields you search frequently, fields you sort, or fields that you join to fields in other tables in queries. However, indexes can slow down some action queries such as append queries, when the indexes for many fields need to be updated while performing these operations.
The primary key of a table is automatically indexed, and you can't index a field whose data type is OLE Object. For other fields, you should consider indexing a field if all the following apply:
If you think you'll often search or sort by two or more fields at a time, you can create an index for that combination of fields. For example, if you often set criteria for LastName and FirstName fields in the same query, it makes sense to create a multiple-field index on both fields.
When you sort a table by a multiple-field index, Microsoft Access sorts first by the first field defined for the index. If there are records with duplicate values in the first field, Microsoft Access sorts next by the second field defined for the index, and so on.
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