When it comes to optimizing the performance of a SQL Server database, one of the key considerations is the design and usage of indexes. Indexes help improve query performance by allowing the database engine to quickly locate the data needed for a query. However, having duplicate or overlapping indexes can be detrimental to the overall performance of the database.

Duplicate indexes are essentially redundant indexes that provide the same set of columns in the same order. These indexes create unnecessary overhead for the database system, as it has to maintain multiple sets of indexes for the same table. This means that whenever there is an update, delete, or insert operation on the table, the database system has to update all the duplicate indexes, resulting in slower performance.

Overlapping indexes, on the other hand, are indexes that have some columns in common. For example, Index1 may have columns Col1, Col2, and Col3, while Index2 has columns Col1, Col2, Col3, Col4, and Col5. In this case, Index1 is considered overlapping with Index2 because it is a subset of Index2. Having overlapping indexes is inefficient because the database system has to maintain both indexes, even though Index1 is not providing any additional benefit.

To identify and remove duplicate and overlapping indexes in SQL Server, you can use the following script:

WITH MyDuplicate AS (
    SELECT 
        Sch.[name] AS SchemaName,
        Obj.[name] AS TableName,
        Idx.[name] AS IndexName,
        INDEX_COL(Sch.[name] + '.' + Obj.[name], Idx.index_id, 1) AS Col1,
        INDEX_COL(Sch.[name] + '.' + Obj.[name], Idx.index_id, 2) AS Col2,
        -- ... (up to Col16)
    FROM 
        sys.indexes Idx
        INNER JOIN sys.objects Obj ON Idx.[object_id] = Obj.[object_id]
        INNER JOIN sys.schemas Sch ON Sch.[schema_id] = Obj.[schema_id]
    WHERE 
        index_id > 0
)
SELECT 
    MD1.SchemaName,
    MD1.TableName,
    MD1.IndexName,
    MD2.IndexName AS OverlappingIndex,
    MD1.Col1,
    MD1.Col2,
    -- ... (up to Col16)
FROM 
    MyDuplicate MD1
    INNER JOIN MyDuplicate MD2 ON MD1.tablename = MD2.tablename
        AND MD1.indexname <> MD2.indexname
        AND MD1.Col1 = MD2.Col1
        AND (MD1.Col2 IS NULL OR MD2.Col2 IS NULL OR MD1.Col2 = MD2.Col2)
        -- ... (up to Col16)
ORDER BY 
    MD1.SchemaName,
    MD1.TableName,
    MD1.IndexName

It is important to carefully review the results of this script before taking any action. You should test it on a development server first, rather than running it directly on a production environment. If you encounter any issues with the script, feel free to reach out and I will be happy to assist you with any necessary modifications.

By identifying and removing duplicate and overlapping indexes, you can significantly improve the performance of your SQL Server database. It is recommended to regularly review and optimize the indexes in your database to ensure optimal query execution.

Stay tuned for more SQL Server tips and tricks in future blog posts!