SQL Server Query Tuning Series - DBCC SHOW_STATISTICS WITH HISTOGRAM
SQL Server Query Tuning Series - DBCC SHOW_STATISTICS WITH HISTOGRAM
Welcome to the "SQL Server Query Tuning Series," where we dive into the exciting world of query optimization in Microsoft SQL Server. In this video, we will focus on a powerful command called DBCC SHOW_STATISTICS with the HISTOGRAM option. Understanding this command and its associated concepts is crucial for fine-tuning query performance and achieving optimal execution plans in SQL Server.
Statistics in SQL Server
Statistics play a vital role in query optimization and performance tuning. In this section, we will explore the importance of statistics and how they contribute to the decision-making process of the SQL Server query optimizer.
Understanding Statistics
Statistics are metadata objects that provide information about the distribution of data in database tables or indexed views. They help SQL Server estimate the number of rows that match a particular query predicate, influencing the choice of execution plans.
Automatic and Manual Statistics
SQL Server maintains statistics automatically on indexed columns or when the AUTO_CREATE_STATISTICS database option is enabled. However, there are cases where manual statistics creation or updates may be necessary for accurate cardinality estimation.
Types of Statistics
SQL Server supports different types of statistics, including column statistics and index statistics. We will explore the differences between these types and their impact on query performance.
DBCC SHOW_STATISTICS WITH HISTOGRAM
DBCC SHOW_STATISTICS is a powerful command that allows us to examine the details of statistics for a specific table or indexed view. In this section, we will dive into the HISTOGRAM option of DBCC SHOW_STATISTICS and explore its capabilities.
Introduction to DBCC SHOW_STATISTICS
DBCC SHOW_STATISTICS is used to display statistical information about the distribution of data in a specific table or indexed view. We'll discuss the syntax and usage of the command.
Understanding the Histogram
The histogram is a crucial component of statistics that provides a detailed representation of the data distribution in a column. We will delve into the structure and significance of the histogram and its role in query optimization.
Analyzing Statistics with HISTOGRAM
We will explore practical examples of using DBCC SHOW_STATISTICS with HISTOGRAM to analyze statistics and gain insights into the data distribution. This analysis can help identify potential optimization opportunities and guide query tuning efforts.
Query Tuning with HISTOGRAM
In this section, we will explore practical techniques for query tuning using the HISTOGRAM option of DBCC SHOW_STATISTICS.
Understanding Selectivity and Cardinality
The histogram provides information about the selectivity of different ranges of values in a column. By analyzing the histogram, we can gain insights into the selectivity of specific values or ranges, which in turn influences cardinality estimation. We'll discuss how to utilize this information for query tuning.
Identifying Skewed Data Distributions
Skewed data distributions can lead to suboptimal query plans. We'll explore how to identify skewed distributions using the histogram and leverage that information to improve query performance.
Optimizing Query Predicates
The histogram can help identify specific values or value ranges that are highly selective. We can use this information to optimize query predicates and improve query performance by filtering rows more effectively.
Conclusion
In this video, we explored the powerful DBCC SHOW_STATISTICS command with the HISTOGRAM option in SQL Server query tuning. By understanding the histogram and utilizing the detailed information provided by DBCC SHOW_STATISTICS, you can gain valuable insights into the data distribution, identify optimization opportunities, and improve query performance.
Restoring Required Databases for Query Tuning Session - • Video
use [JBSWIKI]
GO
drop table table1
select top (200000) * into table1 from table1_Backup
create clustered index Ix_TEST on Table1(Col1)
select * from table1 where Col5=getdate()
update statistics table1 with fullscan
-- Use DBCC SHOW_STATISTICS with HISTOGRAM
DBCC SHOW_STATISTICS(N'dbo.Table1', _WA_Sys_00000005_53CE1A8C)
WITH HISTOGRAM;
GO
create statistics [Multi_Column_Stats] on dbo.Table1 (Col1,Col2)
DBCC SHOW_STATISTICS(N'dbo.Table1', Multi_Column_Stats)
WITH HISTOGRAM;
GO
drop statistics Table1.[Multi_Column_Stats] Disclaimer:
The views expressed on this Video are mine alone and do not reflect the views of my company or anyone else. All postings on this Video are provided “AS IS” with no warranties, and confers no rights.
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