Have you ever wondered about the performance impacts of using COUNT(*) in your SQL Server queries? In this tutorial, we will explore the behavior of COUNT(*) and its effects on different table structures.

What is COUNT(*)?

COUNT(*) is a function in SQL Server that returns the number of items in a group. It always returns an integer value. You can use it to count the number of rows in a table or to count the number of rows that satisfy a specific condition.

Counting Rows in a Table

To count the number of rows in a table, you can use the following syntax:

SELECT COUNT(*) FROM Schema.Table;

This query will return the total number of rows in the specified table.

Counting Rows with a Filter Condition

If you want to count the number of rows that satisfy a specific condition, you can add a WHERE clause to your query. For example:

SELECT COUNT(*) FROM Schema.Table WHERE Column1 = 5;

This query will return the number of rows where the value of Column1 is equal to 5.

Counting Rows in a Subquery

You can also use COUNT(*) in a subquery. For example:

SELECT Column1,
       Column2,
       (
           SELECT COUNT(*) FROM Schema.TableName WHERE Column3 = 3
       )
FROM Schema.TableName;

This query will return the values of Column1 and Column2, along with the count of rows where the value of Column3 is equal to 3.

Counting Rows with Windowing Functions

COUNT(*) can also be used as part of a windowing function. For example:

SELECT Column1,
       Column2,
       COUNT(*) OVER (PARTITION BY Column2) AS CountColumn
FROM dbo.CountTable;

This query will return the values of Column1 and Column2, along with the count of rows for each distinct value of Column2.

Performance Impacts of COUNT(*)

The performance of COUNT(*) can vary depending on the table structure. Let’s explore a few scenarios:

Counting Rows in a Heap

A heap is a table without a clustered index. When you perform a COUNT(*) on a heap, SQL Server scans the entire table. This can have a significant impact on performance. To improve performance, you can consider creating a nonclustered index on a narrow column that is frequently used for counting.

Counting Rows with a Nonclustered Index

If you create a nonclustered index on a column that is frequently used for counting, SQL Server can perform an index scan instead of scanning the entire table. This can significantly improve the performance of COUNT(*). However, if you include a WHERE clause for a nonindexed column, SQL Server will revert to a table scan.

Counting Rows with a Clustered Index

If you create a clustered index on a table, SQL Server will use the clustered index for counting. The performance of COUNT(*) with a clustered index can be affected by the size of the index. If the clustered index is smaller than other indexes, SQL Server may choose to use it. However, if you include a WHERE clause for a nonindexed column, SQL Server will still perform a table scan.

Conclusion

In this tutorial, we explored the behavior of COUNT(*) in SQL Server and its performance impacts on different table structures. We learned that COUNT(*) can have a significant performance impact when used on a heap, but can be optimized by creating appropriate indexes. We also discovered that COUNT(*) and COUNT(1) behave similarly in terms of performance. By understanding these concepts, you can make informed decisions when using COUNT(*) in your SQL Server queries.

Do you have any other ways you use COUNT()? Share your thoughts in the comments below!

Article Last Updated: 2022-11-16