PostgreSQL Interview Questions and Answers
Commonly regarded as one of the most effective free and open-source database management systems, PostgreSQL is utilised to a great extent by developers and organisations. Because PostgreSQL has a very large number of its’s main features, it is important to have a thorough understanding of basic concepts and functionalities in order to be successful in interviews.
Developing your knowledge of PostgreSQL interview questions is the first step for those who dream of being the potential employees in jobs that involve the use of this robust and fully relational database. Having a firm handle on PostgreSQL interview questions is the one that would make you distinct from the others still you will be able to exhibit your capability of dealing with the highly advanced features that PostgreSQL offers.
Pulling together examples from some of the most sought after PostgreSQL interview questions is the main goal for this guide. This means by providing thorough explanations you will be well prepared to the broken case. Whether you are a beginner or an experienced developer, understanding PostgreSQL interview questions will give you the confidence to tackle database-related challenges. This guide will be fine-tuned with PostgreSQL interview questions which will help you better understand the questions about important PostgreSQL concepts, including common SQL queries and complex database management topics.
Every PostgreSQL interview questions can be conceptualized as a cultural fact: that different people approach different themes of a wide range of topics when considering them. The more PostgreSQL interview questions you have, the better since you will equally develop your highlights and know them all deeply.Here is a broad brief on PostgreSQL interview questions along with their answers.
1. What is PostgreSQL?
- PostgreSQL is RDBMS (Relational Database Management System) which is open-source and object-relational that is designed to work with all types of applications and data types.
- The name, “PostgreSQL,” stands for “Post” + “GreSQL, ” with “Post” referring to the object-relational database technology and “GreSQL” to the underlying co-creator’s name.
2. What are the key features of PostgreSQL?
Some of the most notable features of PostgreSQL are:
- ACID Compliance: Ensures that transactions are processed reliably.
- MVCC (Multi-Version Concurrency Control): Allows multiple transactions to occur simultaneously without affecting the consistency of the data.
- Extensibility: Users can define their custom data types, functions, and operators.
- JSON Support: Native support for storing and querying JSON data.
- Table Inheritance: PostgreSQL allows tables to inherit from other tables, similar to object-oriented programming.
- Full-Text Search: Built-in support for full-text search without additional software.
- Foreign Data Wrappers (FDWs): Allows PostgreSQL to query external data sources like other databases or web services.
- Replication: PostgreSQL supports both synchronous and asynchronous replication for high availability.
- Concurrency Control: It uses MVCC to make sure that one thread to run the task of reading, and other thread to the task of writing at the same time without having to wait for each other.
3. What is the difference between CHAR and VARCHAR in PostgreSQL?
- CHAR(n): A string that has a limited number of characters. If the string has fewer characters than n, it will fill them with empty spaces.
- VARCHAR(n): A string with the number of characters fixed but the space that is actually used to store information is flexible. It does not pad the string with spaces, so it can be more space-efficient when handling a variable length string.
Key Difference:
- CHAR is a fixed-length construction, while VARCHAR is a variable-length sequence.
- PostgreSQL prefers decay which means the use of VARCHAR and if CHAR is used, then spaces will be added until the specified length is reached.
4. What is a Primary Key?
- A Primary Key is Used as a columns (or one set of a group of columns) in a database table that which single handedly determines the a unique identification of each record.
- The primary key may or may not be a combination of one or more columns, the key used to identify the row of the table must have the following properties
- The key must contain unique values and is therefore also always a primary key.
- The key cannot contain NULL values, and is therefore an also primary key.
- The key can be a primary key.
- A primary key ensures that the data listed in the table is correct and free from different entries because it has to be unique.
5. What is a Foreign Key?
- A Foreign Key is a column (or a set of columns) that links one table to another by referencing the primary key of another table.
- The Foreign Key is the foundation of a relational database joining technique and also, the concept of the Foreign Key is an essential part of the sensible database designing.
- It helps maintain referential integrity by ensuring that values in the foreign key column must match valid primary key values in the referenced table.
- By not generating any sort of adjustment, we will not deploy the “integrity-critical” feature of the system to cause the system to be maintained unbroken.
- This prevents data inconsistencies while it ensures that related data is maintained across tables.
6. What is the difference between INNER JOIN and OUTER JOIN?
- INNER JOIN: Brings in the rows that have matching values in both the tables. If the matching row is not found, the row is not included (the JOIN process does not display it).
- OUTER JOIN: Brings in the rows even if there is no match in one of the tables. Consists of three types :
- LEFT OUTER JOIN: It fetches the rows that are in left table and connecting with a row matching from the right table. The non-matching rows from the right table will contain NULL.
- RIGHT OUTER JOIN: It lists the rows of the right table that are joined with a row from the left table, but if the matching rows do not exist in the left table, the non-matching rows from the left table will contain NULL.
- FULL OUTER JOIN: It gets the result of both tables, including the rows with NULLs in case of no match.
Example:
— INNER JOIN Example
SELECT employees.name, departments.name
FROM employees
INNER JOIN departments ON employees.department_id = departments.id;
— LEFT OUTER JOIN Example
SELECT employees.name, departments.name
FROM employees
LEFT OUTER JOIN departments ON employees.department_id = departments.id;
7. What is the use of the EXPLAIN command in PostgreSQL?
- ASC provides the query’s execution plan, which is a graphical representation of the way PostgreSQL will execute.
- It illustrates the criteria of execution, the types of joins used, and whether indices are employed.
- Through analyzing the execution plan, programmers are able to rewrite queries for greater efficiency.
Example:
EXPLAIN SELECT * FROM employees WHERE department_id =1;
8. What is the difference between DELETE and TRUNCATE?
- DELETE: Is used to remove rows from a table if they satisfy the condition. It can be undone by rolling back the transactions. DELETE also invokes triggers which cause performance issues.
- TRUNCATE: The entire data from a table is removed instantly, and none of the triggers are fired. It is non-transactional unless you put it in the BEGIN block and it’s faster than DELETE because there is no log of row deletions.
Key Difference:
- DELETE operates conditionally which makes it slower as it also executes triggers but TRUNCATE is the faster one, however, it removes all the rows without support of rollback unless it is within a transaction.
9. What is VACUUM in PostgreSQL?
- The VACUUM command is an exceptionally useful tool for reclamation of storage, suppression of dead rows, and optimization of the database.
- When rows are deleted or updated in PostgreSQL, the space they previously occupied is not immediately made clear.
- VACUUM is the command that comes to the rescue and it contributes to the repairing of this dead space, thus, making the database more efficient.
- There is also an option to ANALYZE, which collects statistics about the table for the query planner.
Example:
VACUUM ANALYZE;
10. What are the types of JOINs in PostgreSQL?
PostgreSQL has different types of joins to offer:
- INNER JOIN: It produces combined rows diagnosed by the relationship between the matching columns from both tables.
- LEFT JOIN (OUTER): It lists rows from the left table regardless they have no matches and, if there are any matches, from the right table.
- RIGHT JOIN (OUTER): It returns all rows from the right table regardless whether there are matches and, if there are any matches, from the left table.
- FULL JOIN (OUTER): It includes all rows from both tables irrespective of the non-matched rows and; therefore, you need to use NULL values to show nothing seems to be matched.
- CROSS JOIN: Output is the concatenation of both tables.
11. What is the SERIAL data type in PostgreSQL?
- The SERIAL data type is a data type that is used to autogenerate the column of type integer, which is the key column by nature.
- It allows for the automatic creation of an incrementally repeating sequence for each new entry in a table.
Example:
CREATE TABLE employees (
id SERIAL PRIMARY KEY,
name VARCHAR(100)
);
12. What are the different types of indexes in PostgreSQL?
PostgreSQL contains some various indexes such as:
- B-tree: This index is default in PostgreSQL and the most commonly used one.
- Hash: This type of index is used exclusively for equality.
- GiST (Generalized Search Tree): This index type is used for data types, which are complex exhibits, e.g., geometric data.
- GIN (Generalized Inverted Index): This kind of indexing is most appropriate when one needs to index composite data types like arrays or JSONB.
- SP-GiST (Space-partitioned Generalized Search Tree): This kind of index suits nonequal distributions of the value space.
- BRIN (Block Range INdexes): This index is useful while dealing with large tables that have naturally ordered data where only specific blocks need to be examined to find blocks containing the data to index.
13. What is a Foreign Data Wrapper (FDW) in PostgreSQL?
- A Foreign Data Wrapper (FDW) is the PostgreSQL extension, which allows users to work with data from other sources as if it were actually present on the database.
- For example, they can retrieve data from the internet or data services, and integrate it with the existing (local) database by using an FDW in the query.
Example:
CREATE EXTENSION postgres_fdw;
CREATE SERVER foreign_server FOREIGN DATA WRAPPER postgres_fdw OPTIONS (host ‘localhost’, dbname ‘other_db’);
IMPORT FOREIGN SCHEMA public FROM SERVER foreign_server INTO local_schema;
14. How do you handle transactions in PostgreSQL?
- Transaction management in PostgreSQL is done through the use of BEGIN, COMMIT, and ROLLBACK statements.
- A transaction unloads a few operations into a single bundle, that is only fully processed (committed) or fully undone (rolled back) in case an error occurs.
Example:
BEGIN;
UPDATE employees SET salary = salary + 1000 WHERE id = 1;
COMMIT;
15. What is a Subquery in PostgreSQL?
- A subquery is a query that is nested within another query.
- This can be used in places such as the WHERE, SELECT and FROM clauses.
- A sub-query is a way to take the values that are required for the outside query from the sub-queries.
Example:
SELECT name FROM employees WHERE id = (SELECT manager_id FROM departments WHERE name = ‘HR’);
16. What is PostGIS in PostgreSQL?
- PostGIS is an extension for geographical types in PostgreSQL that allows the data model of the WGS 84 coordinate system to be split into creating spatial data using different coordinate systems and the object and other spatial constraints, etc.
- It’s essentially a layer on top of the RDBMS, which allows the user to work with spatial data as if it’s a native data type.
17. What are Triggers in PostgreSQL?
- Triggers are database objects which automatically execute within the database when certain events occur on the database tables or views.
- These events can be insert, update, delete, and these triggers are mainly used for data validation, business rules, workflow, and data auditing.
Example:
CREATE TRIGGER update_timestamp
BEFORE UPDATE ON employees
FOR EACH ROW
EXECUTE FUNCTION update_modified_time();
18. What is pg_dump and how is it used?
- pg_dump is a backup and restore command line tool for PostgreSQL. dump is used to back up a database to a file. dump is a database query/pgsql dump/sql database data dump (dumps a table in ASCII file formats) output a file in the specified location to be able to restore a pgsql database later.
Example:
pg_dump -U username dbname > backup.sql
19. What is REINDEX in PostgreSQL?
- REINDEX is a PostgreSQL command used to rebuild the indexes of a table, index, or schema.
- The reason for using this command is to reduce the comparison to make the changes smooth on the remaining database table and then use copy on the exact desired table versions.
Example:
REINDEX TABLE employees;
20. What is EXCEPT in SQL?
- The EXCEPT operator returns the distinct set of rows selected by the first query but not in the second query.
- In other words, it returns a difference of two queries, i.e. the records that are in the first set (query) and not in the second one.
Example:
SELECT name FROM employees
EXCEPT
SELECT name FROM managers;
