Index Object

Description

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

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

Properties

Clustered property, DistinctCount property, Foreign property, IgnoreNulls property, Name property, Primary property, Required property, Unique property.

Methods

CreateField method, CreateProperty method.

See Also Attributes property, CreateIndex method, Index property, OrdinalPosition property.

Example

This example creates a new Index object, appends it to the Indexes collection of the Employees TableDef, and then enumerates the Indexes collection of the TableDef. Finally, it enumerates a Recordset, first using the primary Index, and then using the new Index. The IndexOutput procedure is required for this procedure to run.

Sub IndexObjectX()

    Dim dbsNorthwind As Database
    Dim tdfEmployees As TableDef
    Dim idxNew As Index
    Dim idxLoop As Index
    Dim rstEmployees As Recordset

    Set dbsNorthwind = OpenDatabase("Northwind.mdb")
    Set tdfEmployees = dbsNorthwind!Employees

    With tdfEmployees
        ' Create new index, create and append Field
        ' objects to its Fields collection.
        Set idxNew = .CreateIndex("NewIndex")

        With idxNew
            .Fields.Append .CreateField("Country")
            .Fields.Append .CreateField("LastName")
            .Fields.Append .CreateField("FirstName")
        End With

        ' Add new Index object to the Indexes collection
        ' of the Employees table collection.
        .Indexes.Append idxNew
        .Indexes.Refresh

        Debug.Print .Indexes.Count & " Indexes in " & _
            .Name & " TableDef"

        ' Enumerate Indexes collection of Employees
        ' table.
        For Each idxLoop In .Indexes
            Debug.Print "    " & idxLoop.Name
        Next idxLoop

        Set rstEmployees = _
            dbsNorthwind.OpenRecordset("Employees")

        ' Print report using old and new indexes.
        IndexOutput rstEmployees, "PrimaryKey"
        IndexOutput rstEmployees, idxNew.Name
        rstEmployees.Close

        ' Delete new Index because this is a
        ' demonstration.
        .Indexes.Delete idxNew.Name
    End With

    dbsNorthwind.Close

End Sub

Sub IndexOutput(rstTemp As Recordset, _
    strIndex As String)
    ' Report function for FieldX.

    With rstTemp
        ' Set the index.
        .Index = strIndex
        .MoveFirst
        Debug.Print "Recordset = " & .Name & _
            ", Index = " & .Index
        Debug.Print "    EmployeeID - Country - Name"

        ' Enumerate the recordset using the specified
        ' index.
        Do While Not .EOF
            Debug.Print "    " & !EmployeeID & " - " & _
                !Country & " - " & !LastName & ", " & !FirstName
            .MoveNext
        Loop

    End With

End Sub

Example (Microsoft Access)

The following example creates a new index on an Employees table:

Sub NewIndex()
    Dim dbs As Database, tdf As TableDef, idx As Index
    Dim fld1 As Field, fld2 As Field

    ' Return reference to current database.
    Set dbs = CurrentDb
    ' Return reference to Employees table.
    Set tdf = dbs.TableDefs!Employees
    Set idx = tdf.CreateIndex("EmployeeNameIndex")
    Set fld1 = idx.CreateField("LastName")
    Set fld2 = idx.CreateField("FirstName")
    idx.Fields.Append fld1
    idx.Fields.Append fld2
    idx.Required = True
    tdf.Indexes.Append idx
    Set dbs = Nothing
End Sub