A Detailed Guide on SQL Query Optimization

Siddharth Sachdeva 08 Jul, 2024
12 min read


SQL Query optimization is defined as the iterative process of enhancing a query’s performance in execution time, disk accesses, and other cost-measuring criteria. Data is an integral part of any application. Access to the data should be as fast as possible to enhance the user experience while using the application.

It is a very critical task. Even slight changes in SQL queries can improve performance drastically. There is no step-by-step guide for the same. In turn, we must use general guidelines for writing queries and which operators to use. Then, check for execution plans, find out which part of the query takes the most time, and rewrite that part in another way. In this article, you will get to learn about the SQl Query Optimization its requirements and how this Optimization is Working.

Learning Objectives:

  • Understand the concept of query processing and the steps involved (parsing, optimization, execution planning).
  • Learn about Query optimization in SQL techniques like indexing, selection, join strategies, etc.
  • Familiarize with different metrics to analyze query performance (execution time, I/O statistics, execution plans).
  • Study guidelines and best practices for writing optimized SQL queries.

This article was published as a part of the Data Science Blogathon.

What is SQL?

SQL (Structured Query Language) is a standard language for accessing, storing, creating, and deleting databases and performing all other data-related operations. There are four types of language in SQL:

  • DDL (Data Definition Language): This includes commands for creating and altering the database schema, such as CREATE, DROP, ALTER, and TRUNCATE.
  • DML (Data Manipulation Language): This includes commands for updating the data in the database. It alters the relation instance, such as SELECT, UPDATE, INSERT, and DELETE.
  • DCL (Data Control Language): This includes commands for access to the database, such as GRANT and REVOKE.
  • TCL (Transaction Control Language): This includes the commands used to manage transactions in the database, such as COMMIT and ROLLBACK.


Introduction to SQL | SQL query Optimizaton

Before going toward the main topic of SQL Query optimization, let us first understand the actual processing of the query:

What is Query Processing?

Query processing is the group of phases associated with extracting data from the database. It includes converting queries written in a high-level language such as SQL query optimizer into a form that the physical-level implementation of the database can understand, SQL query optimization techniques, and the original evaluation of the query.

Major Steps in Query Processing

There are three major steps involved in query processing:

Query Processing | SQL Query Optimization


1. Parser and translator: The first step in query processing is parsing and translation. A parser, just like a parser in compilers, checks the syntax of the query to see whether the relations mentioned are present in the database. A high-level query language such as SQL query optimizer is suitable for human use. However, it is unsuitable for system internal representation. Therefore, translation is required. The internal representation can be an extended form of relational algebra.

2. Optimization: An SQL query optimizer can be written in many ways. Its optimization also depends on how the data is stored in the file organization. A Query can also have different corresponding relational algebra expressions.

conditional query | sql query optimization

So, the above query can be written in the two forms of relational algebra. So it depends on the implementation of the file system which one is better.

3. Execution plan: A systematic step-by-step execution of primitive operations for fetching database data is called a query evaluation plan. Different evaluation plans for a particular query have different query costs. The cost may include the number of disk accesses, CPU time for executing the query, and communication time in the case of distributed databases.

What is a Query optimization in SQL?

Query optimization in SQL is enhancing the performance of a database query by modifying the query’s execution plan without altering its end result. The main goal is to reduce the time and resources required to retrieve the requested data from the database. Query optimization in SQL is crucial for ensuring efficient database operations, especially as the size of databases and the complexity of queries increase.

How Does Query Optimization Work?

Query optimization in SQL involves several steps and techniques that a database management system (DBMS) uses to determine the most efficient query execution. Here’s a detailed look at how this process works:

  1. Parsing: When a query is submitted, the DBMS parses it to check for syntax errors and understand the query structure.
  2. Translation: The parsed query is then translated into an internal representation, often an abstract syntax tree (AST) or a similar data structure.
  3. Logical Plan Generation: The DBMS generates a logical query plan, a high-level description of the operations required to execute the query. This plan includes operations like joins, selections, and projections but doesn’t consider the physical aspects of data storage.
  4. Logical Optimization: The logical query plan is optimized using various rules and transformations. This phase may involve predicate pushdown, query rewriting and eliminating redundancies. 
  5. Physical Plan Generation: The optimized logical plan is converted into one or more physical query plans. Each physical plan specifies the methods and algorithms for data access and manipulation, such as index usage, joining algorithms, and access paths. 
  6. Cost Estimation: The DBMS estimates the cost of each physical plan using a cost model that considers factors like I/O operations, CPU usage, and memory usage. The cost estimation involves:
    • Statistics Analysis: Using statistics about the data (e.g., table size, index cardinality, data distribution) to predict the costs.
    • Heuristics: Applying heuristic rules based on typical database performance characteristics.
  7. Plan Selection: The DBMS selects the physical plan with the lowest estimated cost. This plan is considered the optimal execution plan for the query.
  8. Execution: The selected execution plan is executed by the DBMS. During execution, the DBMS may also perform run-time optimizations, such as dynamic memory allocation and adaptive query processing.

Techniques and Tools For Query Optimization

Several techniques and tools are employed in query optimization:

  • Heuristics-Based Optimization: Relies on predefined rules and patterns to transform and optimize queries. It is fast but may not always produce the best possible plan.
  • Cost-Based Optimization: Uses a detailed cost model and database statistics to estimate and compare the costs of different execution plans. It tends to be more accurate but computationally intensive.
  • Materialized Views: Pre-computed views that store query results and can be reused to speed up query processing.
  • Query Hints: Directives provided by users to influence the optimizer’s choices, such as forcing the use of a specific index or join method.
  • Adaptive Query Processing: Dynamically adjusts the query execution plan based on actual run-time conditions and performance metrics.

Also Read: Alibaba’s LLM-R2: Revolutionizing SQL Query Efficiency

Purpose of SQL Query Optimization

The major purposes of SQL Query optimization are:

1. Reduce Response Time

The major goal is to enhance performance by reducing the response time. Minimize the time difference between users requesting data and receiving responses for an improved user experience.

2. Reduced CPU execution time

The CPU execution time of a query must be reduced so that faster results can be obtained.

3. Improved Throughput

Minimize the number of resources accessed to fetch all necessary data. Fetch the number of rows in a particular query in the most efficient manner to use the least resources.

Requirement For SQL Query Optimization

Enhanced efficiency: Optimized queries can significantly speed up data retrieval, particularly on extensive databases. This could result in an improved user experience for apps that depend on the database.

Costs can be minimized by paying for the resources used if your database is on a cloud platform. Optimizing your queries can help decrease resource usage and cut costs.

Scalability is increasingly crucial as your database expands, requiring efficient queries. Efficient searches can manage bigger data sets with minimal decrease in performance.

Database for the Tutorial

This tutorial will use the AdventureWorks database to show various commands and their optimized solutions. You can download the database from here.

AdventureWorks database is a sample database provided by Microsoft SQL Server. This is a standard database showing day-to-day transaction processing for a business. Scenarios include sales, customer management, product management, and human resources.

For more information on the tables and relationships of the database, you can visit this link.

Metrics for Analyzing Query Performance for SQL Query Pptimizer

There are several metrics for calculating the cost of the query in terms of space, time, CPU utilization, and other resources:

1. Execution Time:

The most important metric to analyze the query performance is the execution time of the query. Execution time/Query duration is when the query returns the rows from the database. We can find the query duration using the following commands:

SELECT * FROM SalesLT.Customer;
Execution Time | SQL Query Optimization

By using STATISTICS TIME ON, we can see the parse time, compile-time, execution time, and completion time of the query.

Parse and Compile Time:  The time taken to parse and compile the query to check the syntax of the query is termed  Parse and Compile time.

Execution Time: The query’s CPU time to fetch the data is termed Execution time.

Completion time: The exact time the query returned the result is termed Completion time.

  By analyzing these times, we can get a clear picture of whether the query is performing up to the mark or not.

2. Statistics IO: 

IO is the major time spent accessing the memory buffers for reading operations in case of query. It provides insights into the latency and other bottlenecks when executing the query. By setting STATISTICS IO ON,  we get the number of physical and logical reads performed to execute the query.


SELECT * FROM SalesLT.Customer;
Statistics IO | SQL Query Optimization

Logical reads: Number of reads that were performed from the buffer cache.

Physical reads: The number of reads that were performed from the storage device because they were not available in the cache.

3. Execution Plan: 

An execution plan is a detailed step-by-step processing plan the optimizer uses to fetch the rows. It can be enabled in the database using the following procedure. An execution plan helps us analyze the major phases in a query’s execution. We can also determine which part of the execution takes more time and optimize that sub-part.

SELECT p.Name, Color, ListPrice FROM SalesLT.Product p

INNER JOIN SalesLT.ProductCategory pc

ON P.ProductCategoryID = pc.ProductCategoryID;
Product Category | SQL Query Optimization

As seen above, the execution plan displays which tables were accessed and which index scans were performed to fetch the data. If joins are present, it illustrates how these tables were merged.

Further, we can see a more detailed analysis of each sub-operation performed during query execution. Let us see the analysis of the index scan:

index anaysis | sql query optimization

As seen above, we can obtain the values of the number of rows read, the actual number of batches, the estimated operator cost, the estimated CPU cost, the estimated subtree cost, the number of executions, and actual rebinds. This gives us a detailed overview of the cost involved in query execution.

SQL Query Optimization Techniques

Till now, we have seen how a query is executed and different measures to analyze the query performance. Now, we will learn the techniques to optimize the query performance in SQL. There are some useful practices to reduce the cost. However, the process of optimization is iterative. One needs to write the query, check query performance using io statistics or execution plan, and optimize it. This cycle needs to be followed iteratively for query optimization. The SQL Server finds the optimal and minimal plan to execute the query.


An index is a data structure that provides quick access to the table based on a search key. It helps minimize the disk access required to fetch the rows from the database. An indexing operation can be a scan or a seek. An index scan traverses the entire index for matching criteria, whereas an index seeks filter rows on a matching filter.

For example,

SELECT p.Name, Color, ListPrice FROM SalesLT.Product p
INNER JOIN  SalesLT.ProductCategory pc
ON P.ProductCategoryID = pc.ProductCategoryID
INNER JOIN  SalesLT.SalesOrderDetail sod
ON p.ProductID = sod.ProductID
WHERE p.ProductID>1
SQL Query Optimization Techniques | Indexing


In the above query, 99% of the query execution time goes into the index seek operation, which is an important part of the optimization process.

Guidelines for choosing an index:

  • Make indexes on keys that frequently occur in the WHERE clause and join statements.
  • Do not create indexes on columns that are frequently modified, such as columns that are frequently updated by the UPDATE command.
  • Indexes should be made on Foreign keys where INSERT, UPDATE, and DELETE are concurrently performed. This allows UPDATES on the master table without shared locking on the weak entity.
  • Create indexes on attributes that commonly occur together in the WHERE clause using the AND operator.
  • Indexes should be made on ordering key values.


Follow the selection of the required rows instead of selecting all the rows. SELECT * is highly inefficient as it scans the entire database.

SELECT * FROM SalesLT.Product
SQL Query Optimization Techniques | selection
SELECT ProductNumber, Name, Color,Weight FROM SalesLT.Product
SQL Query Optimization Techniques | selected values

As we can see from the above two outputs, using the SELECT statement to choose only the required columns reduces the time to one-fourth.


The SELECT DISTINCT command in SQL fetches unique results and removes duplicate rows in the relation. To achieve this task, it groups related rows together and removes them. GROUP BY operation is a costly operation. So, to fetch distinct rows and remove duplicate rows, one might use more attributes in the SELECT operation.

Let us take an example,

SELECT DISTINCT Name, Color, StandardCost, Weight FROM SalesLT.Product
SQL Query Optimization Techniques | avoid select distinct
SELECT  Name, Color, StandardCost, Weight, SellEndDate, SellEndDate FROM SalesLT.Product
SQL Query Optimization Techniques | statistics time

As we can see from the execution of the above two queries, the DISTINCT operation takes more time to fetch unique rows. So, it is better to add more attributes to the SELECT query to improve performance and get unique rows.

Inner joins vs. WHERE clause

We should use an inner join to merge two or more tables rather than the WHERE clause. WHERE clause creates the CROSS join/ CARTESIAN product for merging tables. CARTESIAN product of two tables takes a lot of time.

SELECT p.Name, Color, ListPrice 
FROM SalesLT.Product p, SalesLT.ProductCategory pc
WHERE P.ProductCategoryID = pc.ProductCategoryID
 inner vs where clause
SELECT p.Name, Color, ListPrice FROM SalesLT.Product p
INNER JOIN  SalesLT.ProductCategory pc
ON P.ProductCategoryID = pc.ProductCategoryID
SQL Query Optimization Techniques | product category id

So, the above outputs show that an inner join takes almost half the time of a join using a WHERE clause.

LIMIT command

The limit command controls the number of rows to be displayed from the result set. The result set must display only the required rows. Therefore, one must use a limit with the production dataset and provide an on-demand computation of rows for the production purpose

SELECT Name, Color, ListPrice 
FROM SalesLT.Product

The above query prints the top 10 rows of the resultset. This drastically improves the performance of the query.

IN versus EXISTS

IN operator is more costly than EXISTS regarding scans, especially when the subquery result is a large dataset. So, we should try to use EXISTS rather than IN to fetch results with a subquery.

Let us see this with an example,

SELECT ProductNumber,Name,Color FROM SalesLT.Product
(SELECT ProductID FROM SalesLT.ProductDescription)
SELECT ProductNumber,Name,Color FROM SalesLT.Product
(SELECT ProductID FROM SalesLT.ProductDescription)
select while exists

We have executed the same query having a subquery with IN command and EXISTS commands and we observe that the EXISTS command takes half of the time as compared to IN command and the number of physical and logical scans is very low.

Loops versus Bulk insert/update

Loops must be avoided because they require running the same query many times. Instead, we should opt for bulk inserts and updates.


SET @Counter=1
WHILE ( @Counter <= 10)
    PRINT 'The counter value is = ' + CONVERT(VARCHAR,@Counter)
INSERT INTO [SalesLT].[ProductDescription]
           ('This is great'
    SET @Counter  = @Counter  + 1

   Loops versus Bulk insert/update
USE [AdventureWorksLT2019]
INSERT INTO [SalesLT].[ProductDescription]
           ('This is great'
		   ('New news'
		   ('Awesome product.'
		    ('Awesome product.'

As we have seen above, bulk insert works faster than loop statements.


SQL query optimizer is a crucial process for ensuring the efficient performance of database systems. By enhancing query execution time, reducing resource consumption, and improving the overall system scalability, optimization plays a key role in delivering a seamless user experience. This detailed guide has outlined the fundamental concepts and techniques for SQL query optimization, from understanding query processing to implementing practical optimization strategies. Hope you like this article you get to know about the SQL query optimization techniques what are they and how it works.

Key Takeaways:

  • Query optimization aims to enhance query performance by reducing execution time, disk accesses, and resource usage.
  • Indexing tables/columns used in filters, joins, etc. is crucial for optimizing queries.
  • Using appropriate join strategies (INNER JOIN over WHERE clause) and avoiding inefficient operations (DISTINCT, loops) improves efficiency.
  • Analyzing query execution plans, I/O statistics, and execution times helps identify bottlenecks for optimization.

So, we learned how minor query changes can drastically improve their performance. This will boost the performance of applications and provide a better user experience. Keep all the guidelines in mind while writing queries.

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Frequently Asked Questions

Q1. What are the three major steps in query optimization?

A. The three major steps in query optimization are parsing and translation, optimization, and execution planning. These steps transform a high-level query into an efficient execution plan.

Q2. What is optimizing queries with explain?

A. Optimizing queries with EXPLAIN involves using the EXPLAIN command to analyze a query’s execution plan. It helps identify inefficiencies, allowing adjustments for improved performance.

Q3. Why do you need to optimize a query?

A. Optimizing a query is essential to enhance performance, reduce execution time, and minimize resource usage. Efficient queries ensure faster data retrieval and better application responsiveness.

Q4. What is the main task of the query optimizer?

A. The main task of the query optimizer is to determine the most efficient execution plan for a query. It evaluates different strategies to minimize resource usage and execution time.

Siddharth Sachdeva 08 Jul, 2024

Computer science enthusiast

Frequently Asked Questions

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Responses From Readers



It's nice and informative...

NNM 06 Apr, 2023

Good content. Is there End to end SQL tutorial in youtube?

Piyush Sunny
Piyush Sunny 08 Mar, 2024

Thank you for curating this and sharing it with us. It was a good read. Can you also share how can we check the time details in other databases as the one you have used seems to be for sql server studio. Once again, thanks for sharing the content.

Priya Ramakrishnan
Priya Ramakrishnan 26 Mar, 2024

I tried executing the queries using an inner join and then a where clause, the execution plans of which provided no difference. In your case, the results of the query must've been in the cache and hence delivered a faster response in case of an inner join. Please check again