PostgreSQL Interview Questions

1) What is PostgreSQL?

Ans: PostgreSQL is a popular Object-relational database management system. It is an open-source platform that is used to create advanced applications. Familiarity with Linux and UNIX can be an additional benefit when running PostgreSQL.

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2) List some PostgreSQL features.

Ans: PostgreSQL offers a number of helpful features. Some of them are as follows:

• Data integrity protection helps users create a fault-tolerant environment.
• Easily compatible with important platforms, middleware and languages.
• It supports Multi-version concurrency control.
• It supports customer-server network architecture.
• Log-based and Trigger-based replication SSL.
• Highly available and standby server

3) List various data types in PostgreSQL.

Ans: PostgreSQL supports various data types. Some of them are:

• UUID
• Numeric types 
• Temporal types 
• Geometric primitives 
• Arbitrary precision numeric
• XML 
• Boolean
• Character types
• Array
• JSON

4) List some advantages and disadvantages of PostgreSQL.

Ans: Advantages of PostgreSQL:

• Reliable
• Flexible
• Stable
• Easy to learn
• Extensible
• Cross-Platform
• Open-source
• Better Support
• Designed for High Volume Environments

Disadvantages of PostgreSQL:

• It is slower compared to MySQL in terms of performance.
• It does not have support for many open source applications in comparison with MySQL.
• With a greater focus on compatibility, modifications to enhance speed require additional work.

5) What are string Constants?

Ans: The string constant consists of a sequence of characters tied by single quotes('). It is used when inserting a character or passing a character to database objects. PostgreSQL enables the use of single quotes but is integrated by a C-style backslash. It is used in data parsing. 

Example: 'It is an example for a string constant'.

6) What is Multi-Version Control?

Ans: Multi-Version Concurrency Control in PostgreSQL is used to prevent unnecessary database locking. It eliminates the delay time for the user logging into their database. It occurs when some other people are accessing the content. All these transactions are captured.

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7) What are the indices?

Ans: PostgreSQL offers a number of index types: Hash, B-tree, GiST, SP-GiST, BRIN and GIN. Each type of index will use a different algorithm which is most suitable for different types of queries. CREATE INDEX will create B-tree indexes by default that match the most popular situations. Users can set their PostgreSQL indexes as well.

8) What are tokens?

Ans: PostgreSQL tokens are the building blocks for any source code. They include a lot of special character symbols. A token represents an identifier, a quoted identifier, a keyword, a special character symbol or a literal symbol.

9) What is table partitioning?

Ans: In PostgreSQL, table partitioning is a process in which a large table is divided into smaller pieces called partitions. PostgreSQL supports ranges and lists partitioning through table inheritance. Users must create every partition like a child table in the main table.

10) How can we start and stop a database server on PostgreSQL? And how can we check whether PostgreSQL is up and running?

Ans: Prior to accessing the database, you should be able to start the database server. The database server program is referred to as Postgres. The Postgres program needs to know where to look for the data it should use. This is accomplished using the -D option. So the easiest way of starting the server is:

• usr/local/etc/rc.d/010.pgsql.sh start
• /usr/local/etc/rc.d/postgresql start

We use the following way to stop the server:

• /usr/local/etc/rc.d/010.pgsql.sh stop
• /usr/local/etc/rc.d/postgresql stop

We can verify if the postgresql server is up and running by:

• /usr/local/etc/rc.d/010.pgsql.sh status
• /usr/local/etc/rc.d/postgresql status?

11) How is pgadmin used in PostgreSQL?

Ans: Pgadmin is a well-known feature for forming a front-end graphic administration tool. This functionality is available as free software with an artistic license. The latest database administration tool available with an artistic license is Pgadmin iii. It is used to retrieve information, develop, test and continuous maintenance of databases.

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12) Compare PostgreSQL with MySQL.

Ans: MySQL is the "user-friendly, web developer" database, while PostgreSQL is the "feature-rich and standards-compliant" database. PostgreSQL is freely licensed and part of its community; MySQL is GPL licensed and belongs to Oracle. Beyond this, every user of the database should make their own assessment; free software facilitates comparisons.

13) How can we change the column data type in PostgreSQL?

Ans: Using the ALTER TABLE command, we can change a column type.

For example: 

ALTER TABLE name_of_the_table

ALTER COLUMN name_of_the_column [SET DATA] TYPE new_data_type;

14) How are statistics updated within Postgresql?

Ans: In PostgreSQL, To update your statistics, a special function known as an explicit "vacuum" call is done. The way to do this is to create a Vacuum where the Analyze option is used to update the stats in Postgresql as shown:

VACUUM ANALYZE;

15) What is the enable-debug command in PostgreSQL?

Ans: The enable-debug command allows you to compile all apps and libraries. Executing this process usually prevents the system, but it also increases the size of the binary file. The debug symbols that are present usually help developers identify bugs and other issues that may occur related to their script.

16) Which option can be used in PostgreSQL to view the affected rows in the earlier parts of the transaction?

Ans: The SQL standard is defined by four levels of isolating transactions concerning essentially three phenomena. The three phenomena should be avoided between concurrent transactions. Unwanted phenomena include:

Non-repeatable read: A transaction that will read the data which is already read and then discovers that the data has already been altered by another transaction.

Phantom read: A transaction that will execute a query again by returning a set of rows that meets the search condition and finds the set of rows which satisfied the requirements that have changed as a result of another recently initiated transaction.

Dirty read: A transaction that reads the data which is written by a simultaneous uncommitted transaction is called the dirty read.

17) Briefly describe the functions of Postgresql.

Ans: Functions are an important part of running the code on the server everywhere. Some languages used for programming functions are PL/pgSQL, a native PostgreSQL language, as well as other scripting languages such as Perl, PHP, Python, etc. A statistical language called PL/R can also be used to make functions more efficient.

18) What are the different improvements in the relational data model through PostgreSQL?

Ans: A number of improvements have been made to the simple relationship data model through PostgreSQL. They support arrays that include various values, functions, inheritance, and extensibility. Jargon is different in its object-oriented nature in which tables are known as classes.

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19) What kind of database administration tools do you use in Postgresql?

Ans: We have a variety of data administration tools. They are Pgadmin, Psql, Phppgadmin. Phppgadmin is the most popular among them. Most of these tools are web-based interfaces and front-end administration tools.

20) What is Ctid?

Ans: CTIDs is a field that exists in all PostgreSQL tables and is known to recognize specific physical rows based on their block as well as offset positions in a particular table. They are used for index entries for specifying physical rows. It is unique for every record in the table and easily indicates the position of a tuple. The CTID of a logical row changes at the time of its update, such that the CTID will not be used as a long-term row identifier. However, it may be helpful to identify a line within a transaction where no concurrent updates are expected.

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21) Explain about write-ahead logging.

Ans: Write-ahead logging is a standard way of ensuring data integrity. It is a correct rule or protocol for writing actions and modifications in a transaction log. This feature enhances the reliability of the database by saving modifications before any modification or update of the database. It provides the database log if the database crashes. It allows work to begin from the moment it was interrupted.

22) When should PostgreSQL be avoided?

Ans: At the cost of speed, PostgreSQL was built to be expandable and compatible. PostgreSQL may not suit you if your project needs the fastest read operations possible.

Due to its wide range of features and its high adherence to the SQL standard, Postgres may be excessive for simple database configurations. MySQL is the practical choice for operations like read-heavy where we need speed. 

While PostgreSQL provides solid support for replication, it is always a relatively new functionality, and certain configurations like primary architecture are only possible with expansions. Replication is an advanced feature of MySQL. Most users feel that replicating MySQL is easier to implement, especially for those without the required experience in administering databases and systems.

23) What do you mean by Full-Text search in PostgreSQL?

Ans: This refers to the technique or method of searching for a single or a collection of documents stored on a computer in a full-text database. It may be simply distinguished from searches according to metadata or portions of the original texts depicted in the databases. While PostgreSQL is not as advanced as SOLR and Elasticsearch, these two tools are specific to full-text search. While in PostgreSQL, the full-text search is just a feature, and it's quite good.

24) What are BRIN indices?

Ans: Once the BRIN index has been installed, PostgreSQL will be able to select the minimum and maximum values by reading the selected column for each 8k page of data saved. After that, PostgreSQL will store the page number, the maximum and minimum values of the selected column in the BRIN indexes.

25) What is a Parallel Query?

Ans: A parallel query is a PostgreSQL feature in which it can design query plans that can leverage a number of CPUs to respond more quickly to queries.

26) What are GiST indexes in PostgreSQL?

27) What is the Atomicity property's function in PostgreSQL?

Ans: The Atomicity property guarantees the success of all operations within one work unit.

28) Does Postgresql be executed on Cloud?

Ans: Yes. Like the other open source databases, PostgreSQL can be easily executed in virtual containers and is very portable. Many companies like GoGrid, Heroku and Joyent have PostgreSQL support within Cloud hosting environments.

29) How can we perform queries through multiple databases?

Ans: There is no better way to query a database other than the present one. As PostgreSQL loads database-specific system catalogs, we don't know how a cross-database query should behave. contrib/dblink enables cross-database queries with function calls. Of course, a customer can also establish concurrent connections to various databases and merge client-side results.

30) Can we create a shared storage PostgreSQL server cluster?

Ans: PostgreSQL will not support clustering by using shared storage on SCSI, SAN, iSCSI volume, or other shared media. Such "RAC-style" clustering is not supported. Only the replicated clustering is supported at present.

Final Words:

In this blog, we have provided the most frequently asked PostgreSQL Interview questions by the top companies. These are not the mandatory questions to be asked in your interview, but they have more chances to be asked and learning them may help you to clear your PostgreSQL interview. We have provided you with these questions to help you to understand the pattern and how to answer them correctly. Having a better approach and the right way to answer questions may help you clear your interview. You could go through them and advance your career.