Writing SQL queries makes up a significant part of a database administrator’s (DBA) or developer’s job. The ability to write efficient queries can have a dramatic effect on database performance. This post will look at how using basic SQL Server indexes effectively contributes to improved query response time.
It’s easy to write slow queries using SQL. You only tell the database what you want without any explicit instructions on how to satisfy the query. Even simple and similar queries can use different execution plans and take substantially different amounts of time to complete.
The way the data is stored greatly impacts the ability of a database engine to satisfy a query request. Finding records in a database can be accomplished by scanning, seeking, or a combination of both techniques. Using indexes provides a more efficient method of locating specific data elements stored in a database.
How tables are constructed influences the way the database searches for query results. Tables can be structured as a heap in which there is no defined order to its data rows. In some cases, using a heap can be a valid way to store data.
Tables can also have a defined order for data rows and store them based on key values. This structure is known as a Clustered Index (CI) and results in a clustered table. A table can only have one CI. Tables can also have nonclustered indexes which can facilitate finding data elements quickly.
In the vast majority of applications, a clustered index provides more efficient access to specific data elements and improves query performance over using a heap.\
Indexes are structures associated with a table or view designed to speed data retrieval. The index contains keys built from the table’s columns and is stored in a tree. Traversing the tree to locate data elements leads to better database response.
Two types of indexes are available in SQL Server.
Both types of indexes can be unique, meaning that there is only one combination that leads to a specific data element.
Query response time is directly related to the number of pages that need to be read into memory to satisfy a request. Constructing queries that allow the database engine to take advantage of indexes limits the number of pages that need to be examined to return the appropriate results.
Following are some tips for writing queries that make efficient use of indexes.
More tips are available in an IDERA webcast that takes a deeper look into how using indexes properly can improve query performance. The video provides many hands-on examples of using indexes in SQL Server and is an excellent source of information for DBAs and developers interested in improving database performance.
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