19 Query Optimization Interview Questions and Answers

Query optimization is a process that helps improve the performance of a database by reducing the amount of time it takes to execute a query. This is done by identifying and removing bottlenecks that can slow down the query execution process. If you’re interviewing for a position that involves working with databases, it’s likely that you’ll be asked questions about query optimization. In this article, we’ll review some of the most common query optimization questions and how you can answer them.

Query Optimization Interview Questions and Answers

Here are 19 commonly asked Query Optimization interview questions and answers to prepare you for your interview:

1. What is query optimization?

Query optimization is the process of improving the performance of a database query. This can be done by various means, such as improving the query itself, improving the indexing of the database, or improving the database design.

2. Explain what the main components of a relational database are and how they interact with each other?

The main components of a relational database are the tables, which store the data, and the relations, which define the relationships between the data in the tables. The relations are typically defined using foreign keys, which are used to link data in one table to data in another table.

3. Can you explain what an index is in SQL?

In SQL, an index is a data structure that is used to improve the performance of database queries. Indexes can be created on one or more columns of a table, and they allow the database to quickly find the data that is being requested by a query. Indexes can be created using various algorithms, and the choice of algorithm can impact the performance of the index.

4. Can you explain why we need to normalize data?

Normalization is the process of organizing data in a database so that it meets certain requirements, in order to reduce redundancy and improve data integrity. Normalization typically involves breaking down a large table into smaller, more manageable tables, and then linking them together using relationships. This helps to minimize data duplication and ensures that data is stored in a consistent format.

5. How do you optimize stored procedures in a DBMS like Oracle, MySQL, or Postgres?

There are a few different ways that you can optimize stored procedures in a DBMS. One way is to make sure that your procedures are well-indexed. Another way is to use caching techniques to improve performance. Finally, you can also use query optimization tools to help you identify and fix any potential bottlenecks in your procedures.

6. What are some typical ways that the optimizer decides which query plan it should use?

The optimizer typically looks at the cost of each query plan and chooses the one that is cheapest. It may also consider other factors such as the number of rows that need to be retrieved, the order in which the rows need to be retrieved, and the availability of indexes.

7. What is dynamic programming?

Dynamic programming is a technique for solving problems by breaking them down into smaller subproblems, solving each of those subproblems only once, and storing the solutions to each subproblem so that they can be reused later. This technique can be used to solve problems that would be otherwise too difficult to solve by brute force methods.

8. What’s the difference between serialization and deserialization?

Serialization is the process of converting data into a format that can be stored or transmitted. Deserialization is the process of converting data back into its original form.

9. How can you optimize a join statement in SQL?

One way to optimize a join statement is to use a hash join. This type of join can be more efficient than a nested loop join, especially if the data being joined is large. Another way to optimize a join statement is to make sure that the columns being joined are indexed. This can help the join operation to run more quickly.

10. What is the first thing you would do when diagnosing performance issues on a new project?

The first thing I would do when diagnosing performance issues on a new project is to take a look at the query execution plan. This will give me a good idea of where the bottlenecks are in the system and where I need to focus my optimization efforts.

11. What problems can poorly written queries cause?

Poorly written queries can cause a number of problems, including decreased performance, increased resource usage, and incorrect results.

12. Is it possible to enforce referential integrity in MongoDB? If yes, then how?

Yes, it is possible to enforce referential integrity in MongoDB. This can be done by creating a foreign key index on the collection that is being referenced.

13. How does one go about optimizing a slow view running against a large table?

There are a few things that can be done in order to optimize a slow view running against a large table. One is to make sure that the view is only pulling the data that is absolutely necessary, and that all unused columns are removed. Another is to create indexes on the columns that are being used in the view, which can help speed up the query. Finally, it is also important to make sure that the view is using the most efficient query plan possible.

14. What techniques can be used to improve the performance of a Stored Procedure?

There are a number of techniques that can be used to improve the performance of a stored procedure, including:

-Using bind variables to avoid hard parsing
-Using indexes to improve query performance
-Using materialized views to improve query performance
-Using partitioning to improve query performance
-Using compression to reduce the amount of data that needs to be read
-Using the right data type for columns to avoid unnecessary conversions
-Using hints to give the optimizer more information about the data
-Making sure the statistics are up to date

15. Can you give me an example of how you might implement a nested loop join?

A nested loop join is a type of join that is used to compare each row of one table with every row of another table. This can be done by looping through the first table and then, for each row in the first table, looping through the second table to see if there are any matches.

16. Can you explain what an execution plan means?

An execution plan is the set of steps that the database server will take in order to execute a SQL query. The execution plan will take into account the structure of the data, the indexes that are available, and the way that the query is written in order to determine the most efficient way to execute it.

17. Why aren’t statistics being updated on your tables?

Statistics are only updated automatically when the AUTO_UPDATE_STATISTICS database option is turned on. If this option is turned off, then you will need to update statistics manually using the UPDATE STATISTICS command.

18. Can you explain what a deadlock is?

A deadlock is a situation where two or more processes are each waiting for the other to release a resource before they can continue. This can lead to a situation where the processes are effectively locked in a stalemate, unable to continue.

19. What indexes could be created for this query?

In order to optimize the query, we would need to create indexes on the columns that are being queried. In this case, we would create indexes on the ‘name’ and ‘age’ columns.