SQL Performance Tuning

SQL performance tuning is the process of optimizing queries to make database operations faster and more efficient. It reduces response times, minimizes server load, and improves resource utilization.Poorly tuned queries can slow down databases, increase CPU usage, and even cause downtime. Proper tuning ensures quick query execution and smooth overall database performance.

Factors Affecting SQL Speed

Some of the major factors that influence the computation and execution time in SQL are:

• Table Size: Larger tables with millions of rows can slow down query performance if the query hits a large number of rows.
• Joins: The use of complex joins, especially when joining multiple tables, can significantly affect query execution time.
• Aggregations: Queries that aggregate large datasets require more processing time and resources.
• Concurrency: Simultaneous queries from multiple users can overwhelm the database, leading to slow performance.
• Indexes: Proper indexing speeds up data retrieval but, when misused, can lead to inefficiencies.

Ways to Find Slow SQL Queries in SQL Server

1. Creating an Execution Plan

SQL Server Management Studio allows users to view the execution plan, which details how SQL Server processes a query. This plan helps identify inefficiencies like missing indexes or unnecessary table scans. To create an execution plan:

• Start by selecting "Database Engine Query" from the toolbar of SQL Server Management Studio.
• Enter the query after that, and then select "Include Actual Execution Plan" from the Query option.
• It's time to run your query at this point. You can do that by pressing F5 or the "Execute" toolbar button.
• The execution plan will then be shown in the results pane, under the "Execution Pane" tab, in SQL Server Management Studio.

2. Monitor Resource Usage

SQL Server's performance is closely tied to resource usage (CPU, memory, and disk). Monitoring tools like Windows Performance Monitor can track these metrics and highlight performance bottlenecks. We may view SQL Server objects, performance counters, and other object activity with it. Simultaneously watch Windows and SQL Server counters with System Monitor to see if there is any correlation between the two services' performance.

3. Use SQL DMVs to Find Slow Queries

The abundance of dynamic management views (DMVs) that SQL Server includes is one of its best features, helping identify slow-running queries, execution plans, and resource consumption. DMVs such as sys.dm_exec_query_stats can be used to track query performance.

SQL Query Optimization Techniques

Inefficient queries or those containing errors can consume excessive resources in the production database, leading to slower performance or even disconnecting other users. It's important to optimize queries to minimize their impact on overall database performance.

1. SELECT fields instead of using SELECT *

Using SELECT * retrieves all columns from a table, but if you only need specific columns, this can unnecessarily increase processing time. Instead, specify the columns needed. By using the SELECT statement, one may direct the database to only query the data we actually need to suit your business needs.

Example:

Inefficient:

Select * from GeeksTable;

Efficient:

SELECT FirstName, LastName, 
Address, City, State, Zip FROM GeeksTable;

2. Avoid SELECT DISTINCT

It is practical to get rid of duplicates from a query by using SELECT DISTINCT. To get separate results, SELECT DISTINCT GROUPs for every field in the query. However, a lot of computing power is needed to achieve this goal. Instead of using DISTINCT, refine your query to return unique results naturally by adjusting the selection criteria.

Inefficient:

SELECT DISTINCT FirstName, LastName,
State FROM GeeksTable;

Efficient:

SELECT  FirstName, LastName,
State FROM GeeksTable WHERE State IS NOT NULL;

3. Use INNER JOIN Instead of WHERE for Joins

Joining tables using the WHERE clause can lead to inefficiencies and unnecessary computations. It's more efficient to use INNER JOIN or LEFT JOIN for combining tables.

Inefficient:

SELECT GFG1.CustomerID, GFG1.Name, GFG1.LastSaleDate
FROM GFG1, GFG2
WHERE GFG1.CustomerID = GFG2.CustomerID

Efficient:

SELECT GFG1.CustomerID, GFG1.Name, GFG1.LastSaleDate
FROM GFG1 
INNER JOIN GFG2
ON GFG1.CustomerID = GFG2.CustomerID

4. Use WHERE Instead of HAVING

The HAVING clause is used after aggregation and can be less efficient. When possible, use WHERE to filter results before aggregation to speed up the query. A WHERE statement is more effective if the goal is to filter a query based on conditions. Assuming 500 sales were made in 2019, for instance, query to find how many sales were made per client that year.

Inefficient:

SELECT GFG1.CustomerID, GFG1.Name, GFG1.LastSaleDate
 FROM GFG1 INNER JOIN GFG2
ON GFG1.CustomerID = GFG2.CustomerID
GROUP BY GFG1.CustomerID, GFG1.Name
HAVING GFG2.LastSaleDate BETWEEN "1/1/2019" AND "12/31/2019"

Efficient:

SELECT GFG1.CustomerID, GFG1.Name, GFG1.LastSaleDate
FROM GFG1 INNER JOIN GFG2
ON GFG1.CustomerID = GFG2.CustomerID
WHERE GFG2.LastSaleDate BETWEEN "1/1/2019" AND "12/31/2019"
GROUP BY GFG1.CustomerID, GFG1.Name

5. Limit Wildcards to the End of a Search Term

Wildcards enable the broadest search when searching unencrypted material, such as names or cities. However, the most extensive search is also the least effective. Using wildcards like % at the beginning of a string makes it difficult for SQL to efficiently use indexes. It's better to place them at the end of the search term.

Inefficient:

SELECT City FROM GeekTable WHERE City LIKE ‘%No%’

Efficient:

SELECT City FROM GeekTable WHERE City LIKE ‘No%’ 

6. Use LIMIT for Sampling Query Results

Limiting the results using LIMIT can help avoid querying the entire table when first testing or analyzing a query. Only the given number of records are returned by the LIMIT statement. By using a LIMIT statement, we can avoid stressing the production database with a big query only to discover that it needs to be edited or improved.

Query:

SELECT GFG1.CustomerID, GFG1.Name, GFG1.LastSaleDate
FROM GFG1
INNER JOIN GFG2
ON GFG1.CustomerID = GFG2.CustomerID
WHERE GFG2.LastSaleDate BETWEEN "1/1/2019" AND "12/31/2019"
GROUP BY GFG1.CustomerID, GFG1.Name
LIMIT 10

7. Run Queries During Off-Peak Hours

Running heavy queries during off-peak hours reduces the load on the database, minimizing the impact on other users. About planning any query to run at a time when it won't be as busy in order to reduce the impact of our analytical queries on the database. When the number of concurrent users is at its lowest, which is often overnight, the query should be executed.

Index Tuning

When choosing and building indexes, database tuning includes index tuning. The index tuning objective is to speed up query processing. It can be challenging to employ indexes in dynamic contexts with numerous ad-hoc searches scheduled in advance. The queries that are based on indexes are subject to index tweaking, and the indexes are generated automatically as needed. Users of the database do not need to take any specific activities to tune the index.

Points to consider while creating indexes:

• Short indexes for reduced disk space and faster comparisons
• Distinct indexes with minimal duplicates for better selectivity
• Clustered indexes covering all row data for optimal performance
• Static data columns for clustered indexes to minimize shifting

SQL Performance Tuning Tools

Utilizing index tuning tools and following best practices is essential for maintaining high-performing SQL Server environments. Regular monitoring, proactive maintenance, and continuous improvement are key to optimizing database performance and supporting critical business applications.

Several SQL performance tuning tools can help identify and optimize database performance. Some of the popular tools include:

• SQL Sentry (SolarWinds)
• SQL Profiler (Microsoft)
• SQL Index Manager (Red Gate)
• SQL Diagnostic Manager (IDERA)

These tools assist with monitoring, identifying slow queries, and recommending optimization strategies for improving database performance.