Understanding Indexing in Microsoft SQL Server: Best Practices and Tips

This guide will teach you about indexing in Microsoft SQL Server and how they can help optimise your queries for better performance.

Understand the Basics of Indexing in SQL Server

It is important to understand the fundamentals of indexing in SQL Server. An index is a data structure that speeds up data retrieval operations on a database table. It operates by making a copy of the table's selected columns and arranging them in a specific way. This enables the database engine to rapidly discover and obtain the needed data, minimising the need for full table scans. Various indexes, including clustered, nonclustered, and unique, serve a specialised purpose.

Identify the Right Columns to Index

When optimising your Microsoft SQL Server's performance, selecting the proper columns to index is important. Not all columns should be indexed; indexing too many can cause your database to slow down.

Start by examining your queries and noting which columns are commonly used in search criteria or join operations. These columns would be ideal for indexing. Consider the column's cardinality, which relates to the number of distinct values it holds. Columns with a high cardinality are more suited for indexing since they allow for more selective search conditions.

Choose the Right Type of Index

It is important to select the appropriate type of index for your database when optimising the performance of your Microsoft SQL Server. Various index types are available, including clustered, nonclustered, and filtered indexes.

1. Clustered indexes establish the physical order of data in a database, and each table can have only one clustered index.
2. Nonclustered indexes produce a distinct structure, including the indexed columns as well as a link to the real data. A table can have many nonclustered indexes.
3. Filtered indexes are nonclustered indexes that only comprise a portion of a table's rows. It can be handy when you have a huge table with a limited subset of regularly viewed data.

When selecting the proper index type, consider your database's specific needs and the queries commonly conducted. Clustered indexes are widely utilised in columns and are regularly used via sorting and range queries. Columns regularly used in search criteria or join operations benefit from nonclustered indexes. Filtered indexes might be helpful when specific portions of data are often accessed.

Regularly Monitor and Maintain Indexes

Regularly monitoring and maintaining indexes is important for optimising the performance of your Microsoft SQL Server. Over time, when data is added, edited, or deleted, indexes in your database might become fragmented or outdated, resulting in lower query performance.

It is important to check fragmentation levels and update statistics on your indexes frequently to ensure they perform optimally. When the logical order of the index pages does not match the physical order of the data pages, fragmentation occurs, resulting in decreased query performance.

In addition to monitoring fragmentation, it helps keep index statistics current. Statistics provide information about the data distribution in a table to the query optimiser, allowing the SQL server to determine the optimum query execution plans. Outdated statistics can result in poor query performance because the optimiser may make inaccurate assumptions about data distribution.

Use Indexing Tools and Techniques to Optimise Performance

Numerous tools and approaches are available to assist you in optimising indexing performance in Microsoft SQL Server. The Database Engine Tuning Advisor tool may analyse your workload and offer index modifications to increase performance.

Another approach is to use filtered indexes, which allow you to establish an index on a portion of a table's rows. This is especially beneficial for searches that only access a subset of the data.

Consider employing indexed views and precomputed result sets stored in the database. These can boost query performance by allowing the optimiser to use the indexed view instead of costly joins or aggregations.

You should also analyse and change your indexing approach regularly. As your data and workload evolve, so will your indexing requirements. Reviewing and changing your indexes regularly guarantees they continue to work optimally.

Creating Indexes in SQL Server

Here are some examples of scripts for creating clustered and nonclustered indexes in SQL Server.

How To Create Clustered Indexes

In SQL Server, use the 'CREATE CLUSTERED INDEX' query to construct a clustered index. A clustered index specifies the physical order of data rows in a database, and each table can only have one clustered index. The basic syntax for building a clustered index is as follows:

CREATE CLUSTERED INDEX index_name
ON table_name (column1, column2, ...);

Where index_name is a unique name for the clustered index. This is how you'll reference the index later. table_name is the name of the table for which you want to create the clustered index, and (column1, column2, ...) is a list of the columns by which you want to define the clustered index. The data rows in the table will be physically sorted based on these columns.

Keep in mind the following considerations:

1. The clustered index determines the physical order of data in the table, so choose the columns carefully. Typically, columns with unique values are good candidates.
2. Creating or modifying a clustered index can be resource-intensive, especially on large tables. It might require downtime in some cases, so plan accordingly.
3. The choice of a clustered index can significantly impact query performance, so make sure it aligns with your application's query patterns.
4. You can have only one clustered index per table but multiple nonclustered indexes.
5. SQL Server automatically maintains Clustered indexes, so you don't need to manage the physical order of the data manually.

After constructing the clustered index, SQL Server will automatically organise the data rows in the table based on the defined index columns, which may improve query performance for queries that use the same columns in their 'WHERE' clauses or joins.

How To Create Non-Clustered Indexes

Nonclustered indexes are very similar in the way they are created. You can use the `CREATE NONCLUSTERED INDEX` statement to create a nonclustered index in SQL Server. Unlike a clustered index, a table can have multiple nonclustered indexes. Here's the basic syntax for creating a nonclustered index:

CREATE NONCLUSTERED INDEX index_name
ON table_name (column1, column2, ...);

Keep in mind the following considerations:

1. You can have multiple nonclustered indexes on a table to optimise query patterns. However, each additional index requires additional storage space and may have some impact on insert, update, and delete operations.
2. Nonclustered indexes are handy when you need to speed up specific SELECT queries by providing efficient access paths.
3. While nonclustered indexes improve query performance, they don't change the physical order of data in the table. The data remains organised according to the clustered index or, if none exists, in the insertion order.
4. It's essential to carefully choose the columns for your nonclustered index to align with the most commonly used query criteria in your application.