Index Design Basics
Poorly designed indexes and a lack of indexes are primary sources of database application bottlenecks. Designing efficient indexes is paramount to achieving good database and application performance. The selection of the right indexes for a database and its workload is a complex balancing act between query speed and update cost. Narrow indexes, or indexes with few columns in the index key, require less disk space and maintenance overhead. Wide indexes, on the other hand, cover more queries. You may have to experiment with several different designs before finding the most efficient index. Indexes can be added, modified, and dropped without affecting the database schema or application design. Therefore, you should not hesitate to experiment with different indexes.
The query optimizer in SQL Server reliably chooses the most effective index in the vast majority of cases. Your overall index design strategy should provide a variety of indexes for the query optimizer to choose from and trust it to make the right decision. This reduces analysis time and produces good performance over a variety of situations. To see which indexes the query optimizer uses for a specific query, in SQL Server Management Studio, on the Query menu, select Include Actual Execution Plan. For more information, see How to: Display an Actual Execution Plan.
Do not always equate index usage with good performance, and good performance with efficient index use. If using an index always helped produce the best performance, the job of the query optimizer would be simple. In reality, an incorrect index choice can cause less than optimal performance. Therefore, the task of the query optimizer is to select an index, or combination of indexes, only when it will improve performance, and to avoid indexed retrieval when it will hinder performance.
The follow tasks make up our recommended strategy for designing indexes:
Understand the characteristics of the database itself. For example, is it an online transaction processing (OLTP) database with frequent data modifications, or a Decision Support System (DSS) or data warehousing (OLAP) database that contains primarily read-only data? For more information, see Online Transaction Processing vs. Decision Support.
Understand the characteristics of the most frequently used queries. For example, knowing that a frequently used query joins two or more tables will help you determine the best type of indexes to use. For more information, see General Index Design Guidelines.
Understand the characteristics of the columns used in the queries. For example, an index is ideal for columns that have an integer data type and are also unique or nonnull columns. A filtered index is appropriate for columns that have well-defined subsets of data. For more information, see Filtered Index Design Guidelines.
Determine which index options might enhance performance when the index is created or maintained. For example, creating a clustered index on an existing large table would benefit from the ONLINE index option. The ONLINE option allows for concurrent activity on the underlying data to continue while the index is being created or rebuilt. For more information, see Setting Index Options.
Determine the optimal storage location for the index. A nonclustered index can be stored in the same filegroup as the underlying table, or on a different filegroup. The storage location of indexes can improve query performance by increasing disk I/O performance. For example, storing a nonclustered index on a filegroup that is on a different disk than the table filegroup can improve performance because multiple disks can be read at the same time. For more information, see Placing Indexes on Filegroups.
Alternatively, clustered and nonclustered indexes can use a partition scheme across multiple filegroups. Partitioning makes large tables or indexes more manageable by letting you access or manage subsets of data quickly and efficiently, while maintaining the integrity of the overall collection. For more information, see Partitioned Tables and Indexes. When you consider partitioning, determine whether the index should be aligned, that is, partitioned in essentially the same manner as the table, or partitioned independently. For more information, see Special Guidelines for Partitioned Indexes.
For more information about these tasks, see General Index Design Guidelines.