Index Object

Index objects specify the order of records accessed from database tables and whether or not duplicate records are accepted, providing efficient access to data. For external databases, Index objects describe the indexes established for external tables (Microsoft Jet workspaces only).

Remarks

The Microsoft Jet database engine uses indexes when it joins tables and creates Recordset objects. Indexes determine the order in which table-type Recordset objects return records, but they don't determine the order in which the Microsoft Jet database engine stores records in the base table or the order in which any other type of Recordset object returns records.

With an Index object, you can:

• Use the Required property to determine whether the Field objects in the index require values that are not Null, and then use the IgnoreNulls property to determine whether the Null values have index entries.
  
  
• Use the Primary and Unique properties to determine the ordering and uniqueness of the Index object.

The Microsoft Jet database engine maintains all base table indexes automatically. It updates indexes whenever you add, change, or delete records from the base table. Once you create the database, use the CompactDatabase method periodically to bring index statistics up-to-date.

When accessing a table-type Recordset object, you specify the order of records using the object's Index property. Set this property to the Name property setting of an existing Index object in the Indexes collection. This collection is contained by the TableDef object underlying the Recordset object that you're populating.

Note   You don't have to create indexes for a table, but for large, unindexed tables, accessing a specific record or processing joins can take a long time. Conversely, having too many indexes can slow down updates to the database as each of the table indexes is amended.

The Attributes property of each Field object in the index determines the order of records returned and consequently determines which access techniques to use for that index.

Each Field object in the Fields collection of an Index object is a component of the index. To define a new Index object, set its properties before you append it to a collection, making the Index object available for subsequent use.

Note   You can modify the Name property setting of an existing Index object only if the Updatable property setting of the containing TableDef object is True.

When you set a primary key for a table, the Microsoft Jet database engine automatically defines it as the primary index. A primary index consists of one or more fields that uniquely identify all records in a table in a predefined order. Because the primary index field must be unique, the Microsoft Jet database engine automatically sets the Unique property of the primary Index object to True. If the primary index consists of more than one field, each field can contain duplicate values, but the combination of values from all the indexed fields must be unique. A primary index consists of a key for the table and is always made up of the same fields as the primary key.

Important   Make sure your data complies with the attributes of your new index. If your index requires unique values, make sure that there are no duplicates in existing data records. If duplicates exist, the Microsoft Jet database engine can't create the index; a trappable error results when you attempt to use the Append method on the new index.

When you create a relationship that enforces referential integrity, the Microsoft Jet database engine automatically creates an index with the Foreign property, set as the foreign key in the referencing table. After you've established a table relationship, the Microsoft Jet database engine prevents additions or changes to the database that violate that relationship. If you set the Attributes property of the Relation object to allow cascading updates and cascading deletes, the Microsoft Jet database engine updates or deletes records in related tables automatically.

To create a new Index object

1. Use the CreateIndex method on a TableDef object.
   
   
2. Use the CreateField method on the Index object to create a Field object for each field (column) to be included in the Index object.
   
   
3. Set Index properties as needed.
   
   
4. Append the Field object to the Fields collection.
   
   
5. Append the Index object to the Indexes collection.

   Note   The Clustered property is ignored for databases that use the Microsoft Jet database engine, which doesn't support clustered indexes.