Advantage of Indexes in SQL Server

The database engine can use indexes to boost performance in a number of different queries. Sometimes these performance improvements are dramatic. An important feature of SQL Server 2000 is a component known as the query optimizer. The query optimizer's job is to find the fastest and least resource intensive means of executing incoming queries. An important part of this job is selecting the best index or indexes to perform the task. In the following sections we will examine the types of queries with the best chance of benefiting from an index.

Searching For Records

The most obvious use for an index is in finding a record or set of records matching a WHERE clause. Indexes can aid queries looking for values inside of a range (as we demonstrated earlier), as well as queries looking for a specific value. By way of example, the following queries can all benefit from an index on UnitPrice:

DELETE FROM Products WHERE UnitPrice = 1

UPDATE Products SET Discontinued = 1 WHERE UnitPrice > 15

SELECT * FROM PRODUCTS WHERE UnitPrice BETWEEN 14 AND 16

Indexes work just as well when searching for a record in DELETE and UPDATE commands as they do for SELECT statements.

Sorting Records

When we ask for a sorted dataset, the database will try to find an index and avoid sorting the results during execution of the query. We control sorting of a dataset by specifying a field, or fields, in an ORDER BY clause, with the sort order as ASC (ascending) or DESC (descending). For example, the following query returns all products sorted by price:

SELECT * FROM Products ORDER BY UnitPrice ASC

With no index, the database will scan the Products table and sort the rows to process the query. However, the index we created on UnitPrice (IDX_UnitPrice) earlier provides the database with a presorted list of prices. The database can simply scan the index from the first entry to the last entry and retrieve the rows in sorted order.

The same index works equally well with the following query, simply by scanning the index in reverse.

SELECT * FROM Products ORDER BY UnitPrice DESC

Grouping Records

We can use a GROUP BY clause to group records and aggregate values, for example, counting the number of orders placed by a customer. To process a query with a GROUP BY clause, the database will often sort the results on the columns included in the GROUP BY. The following query counts the number of products at each price by grouping together records with the same UnitPrice value.

SELECT Count(*), UnitPrice FROM Products GROUP BY UnitPrice

The database can use the IDX_UnitPrice index to retrieve the prices in order. Since matching prices appear in consecutive index entries, the database is able count the number of products at each price quickly. Indexing a field used in a GROUP BY clause can often speed up a query.

Maintaining a Unique Column

Columns requiring unique values (such as primary key columns) must have a unique index applied. There are several methods available to create a unique index. Marking a column as a primary key will automatically create a unique index on the column. We can also create a unique index by checking the Create UNIQUE checkbox in the dialog shown earlier. The screen shot of the dialog displayed the index used to enforce the primary key of the Products table. In this case, the Create UNIQUE checkbox is disabled, since an index to enforce a primary key must be a unique index. However, creating new indexes not used to enforce primary keys will allow us to select the Create UNIQUE checkbox. We can also create a unique index using SQL with the following command:

CREATE UNIQUE INDEX IDX_ProductName On Products (ProductName)

The above SQL command will not allow any duplicate values in the ProductName column, and an index is the best tool for the database to use to enforce this rule. Each time an application adds or modifies a row in the table, the database needs to search all existing records to ensure none of values in the new data duplicate existing values. Indexes, as we should know by now, will improve this search time.

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Index Drawbacks

There are tradeoffs to almost any feature in computer programming, and indexes are no exception. While indexes provide a substantial performance benefit to searches, there is also a downside to indexing. Let's talk about some of those drawbacks now.

Indexes and Disk Space

Indexes are stored on the disk, and the amount of space required will depend on the size of the table, and the number and types of columns used in the index. Disk space is generally cheap enough to trade for application performance, particularly when a database serves a large number of users. To see the space required for a table, use the sp_spaceused system stored procedure in a query window.

EXEC sp_spaceused Orders

Given a table name (Orders), the procedure will return the amount of space used by the data and all indexes associated with the table, like so:

Name    rows     reserved    data    index_size  unused       
------- -------- ----------- ------  ----------  -------
Orders  830      504 KB      160 KB  320 KB      24 KB

According to the output above, the table data uses 160 kilobytes, while the table indexes use twice as much, or 320 kilobytes. The ratio of index size to table size can vary greatly, depending on the columns, data types, and number of indexes on a table.

Indexes and Data Modification

Another downside to using an index is the performance implication on data modification statements. Any time a query modifies the data in a table (INSERT, UPDATE, or DELETE), the database needs to update all of the indexes where data has changed. As we discussed earlier, indexing can help the database during data modification statements by allowing the database to quickly locate the records to modify, however, we now caveat the discussion with the understanding that providing too many indexes to update can actually hurt the performance of data modifications. This leads to a delicate balancing act when tuning the database for performance.

A Disadvantage to Clustered Indexes

If we update a record and change the value of an indexed column in a clustered index, the database might need to move the entire row into a new position to keep the rows in sorted order. This behavior essentially turns an update query into a DELETE followed by an INSERT, with an obvious decrease in performance. A table's clustered index can often be found on the primary key or a foreign key column, because key values generally do not change once a record is inserted into the database.