PostgreSQL Interview Questions

PostgreSQL Interview Questions

  • 1)  What will be the new characteristics of Postgre 9.1?
  • 2) List different datatypes of Postgresql?
  • 3) What are tokens in PostgreSQL?
  • 4) List different advantages of Postgresql?
  • 5) How are the stats updated in Postgresql?
  • 6) What is multi-version control in PostgreSQL?
  • 7) What are string constants in PostgreSQL?
  • 8) Provide a brief explanation of the functions in Postgresql?
  • 9) What is table partitioning in PostgreSQL?
  • 10) What is Cube Root Operator (||/) in PostgreSQL?
  • 11) How can we change the columns datatype in PostgreSQL?
  • 12) What is PostgreSQL?
  • 13) Compare ‘PostgreSQL’ with ‘NoSQL’?
  • 14) What is difference between clustered index and non clustered index?
  • 15) Explain the history of PostgreSQL?
  • 16) Which are different types of database administration tools used in Postgresql?
  • 17)  What is the command enable-debug in PostgreSQL?
  • 18) Provide an explanation About Write Ahead Logging?
  • 19) What are the Indices of PostgreSQL?
  • 20) List some of the features of Postgresql?
  • 21) Put some light on Multi-Version concurrency control?
  • 22) Do provide an explanation for pgadmin? (100% asked Postgresql Interview Questions)
  • 23) What is the option that can be used in PostgreSQL to make transactions see rows affected in previous parts of the transaction?
  • 24) What is use of pgadmin in PostgreSQL?
  • 25) What Is A Ctid?


Postgresql is a popular Object-Relational Database Management system that is primarily used for large web applications. Every year, there are various openings in this industry at large organisations such as Oracle, SQLite, Amazon Web Services, and others. People that are computer savvy will have no trouble passing the interview.

We’ve compiled a list of some of the most significant and anticipated interview questions. The questions can be understood with a little effort, hard work, and thoroughness. While going over the questions, you may notice several that appear in practically every interview. These well-known questions and responses will ensure that you breeze through the interview without breaking a sweat.

During the process of updating the project, one can never be certain what features will go in and which ones won’t make the cut. The project has precise and stringent standards for quality, and some patches may or may not match them before the set deadline. Currently, the 9.1 version is working on some important features which include JSON support, synchronous replication, nearest-neighbor geographic searches, collations at the column level, SQL/MED external data connections, security labels as well as index-only access. However, this list has a high chance of changing completely by the time Postgre 9.1 is released.

 There are new, different data-types supported by Postgresql. Following are those data-types:

  • UUID
  • Numeric types
  • Boolean
  • Character types
  • Temporal types
  • Geometric primitives
  • Arbitrary precision numeric
  • XML
  • Arrays etc.

Users can also create their indexes and get them indexed.

Tokens in PostgreSQL are the building blocks of any source code. They are known to comprise many of the special character symbols. These can be regarded as constants, quoted identifiers, other identifiers, and keywords. Tokens which are keywords consist of pre-defined SQL commands and meanings. Identifiers are used to represent variable names like columns, tables, etc.

Following are some of the advantages of PostgreSQL :

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

It is not that hard as it seems. To get your statistics updated in PostgreSQL a special function called explicit ‘vacuum’ call is made. The method to do is to create a Vacuum where the option of Analyze is used to update statistics in Postgresql


is the syntax.

Multi-version concurrency control or MVCC in PostgreSQL is used to avoid unnecessary locking of the database. This removes the time lag for the user to log into his database. This feature or time lag occurs when someone else is accessing the content. All the transactions are kept as a record.

string constant in PostgreSQL is a sequence of some character that is bounded by single quotes (‘).


‘This is a string Constant’

 Anywhere, functions are an important part because they help in executing the code on the server. Some of the languages to program functions are PL/pgSQL, a native language of PostgreSQL, and other scripting languages like Perl, Python, PHP, etc. statistical language named PL/R can also be used to increase the efficiency of the functions.

Table partitioning in PostgreSQL is the process of splitting a large table into smaller pieces. A partitioned table is a logical structure used to divide a large table into smaller pieces called partitions.

PostgreSQL Cube Root Operator (||/) is used to get the cube root of a number.


SELECT ||/40 AS "Cube Root of 40";

Use change column type statement with ALTER TABLE command to change a column type in PostgreSQL.


ALTER TABLE table_name
ALTER COLUMN column_name [SET DATA] TYPE new_data_type;

The expression ‘NoSQL’ encompasses a wide collection of implementations which are part of the non-relational database. This includes tiny embedded databases such as TokyoCabinet, massive bunched data processing platforms such as Hadoop and everything in between. In short, it’s practically impossible to comment on the range comprised by NoSQL as a typical class.

Choosing between the non-relational and relational databases is also quite commonly debated as both have existed alongside each other for over forty years. In fact, users should opt for the features, community support and implementation of the database according to their current application needs. Additionally, use of multiple various databases for sizeable projects is becoming more of a norm than a trend. Moreover, the users of PostgreSQL are no exception.

Difference between clustered index and non clustered index

  • Cluster index is an index type that is used to sort table data rows on the basis of their key values. In RDBMS primary key allows us to create a clustered index based on that specific column.
  • non-clustered index (or regular b-tree index) is an index where the order of the rows does not match the physical order of the actual data. It is instead ordered by the columns that make up the index.

The origin of PostgreSQL dates back to 1986 as part of the POSTGRES project at the University of California at Berkeley and has more than 30 years of active development on the core platform. It runs on all the major operating systems and has been ACID-compliant since 2001. It also has add-on like PostGIS database extender. In MAC OS Postgresql is the default database. Michel Stonebraker is Father of Postgresql who has started the Post Ingres project for supporting Contemporary Database systems.PostgreSQL’s developers pronounce PostgreSQL as It is abbreviated as Postgres because of ubiquitous support for the SQL Standard among most relational databases.PostgreSQL, originally called Postgres, was created at UCB by a computer science professor named Michael Stonebraker, who went on to become the CTO of Informix Corporation.

Stonebraker started Postgres in 1986 as a followup project to its predecessor, Ingres, now owned by Computer Associates. The name Postgres thus plays off of its predecessor (as in “after Ingres”). Ingres, developed from 1977 to 1985, had been an exercise in creating a database system according to classic RDBMS theory.  Postgres, developed in 1986-1994, was a project meant to break new ground in database concepts such as exploration of “object-relational” technologies. An enterprise-class database, PostgreSQL boasts sophisticated features such as Multi-Version Concurrency Control (MVCC), point in time recovery, tablespaces, asynchronous replication, nested transactions (savepoints), online/hot backups, a sophisticated query planner/optimizer, and write-ahead logging for fault tolerance.

There are the number of data administration tools, and they are

  • Phppgadmin
  • Psql
  • Pgadmin

Out of these, phppgadmin is the most popular one. Most of these tools are front-end administration tools and web-based interfaces.

The command enable-debug is used to enable the compilation of all the applications and libraries. The execution of this procedure usually impedes the system, but it also amplifies the binary file size. Debugging symbols which are present generally assist the developers for spotting the bugs and other problems which may arise associated with their script.
WAL or write-ahead logging is a standard method to ensure data integrity. It is a protocol or the correct rule to write both actions and changes into a transaction log. This feature is known to increase the reliability of the database by logging changes before any changes or updating to the database. This provides the log of the database in case of a database crash. This helps to start the work from the point it was discontinued.

Indices of PostgreSQL are inbuilt functions or methods like GIST Indiceshash table and B-tree (Binary tree) which can be used by the user to scan the index in a backward manner. Users can also define their indices of PostgreSQL.

 Following are some of the major features of Postgresql :

  1. Object-relational database
  2. Supports major Operating systems
  3. Support Extensibility for SQL and Complex SQL queries
  4. Nested transactions
  5. Flexible API and Database validation
  6. Multi-version concurrency control (MVCC) and Procedural languages
  7. WAL and Client server
  8. Table inheritance & Asynchronous replication
 MVCC or better known as Multi-version concurrency control is used to avoid unwanted locking of the database. The time lag for the user is removed so that one can easily log into his database. All the transactions are well- kept as a record. The time lag occurs when someone else is on the content.
 Pgadmin is a feature that is known to form a graphical front-end administration tool. This feature is available under free software released under Artistic License. Pgadmin iii is the new database administration tool released under artistic license.

 The SQL standard is defined by four levels of transaction isolation basically regarding three phenomena. The three phenomenon must be prevented between concurrent transactions. The unwanted phenomena are:

  • Phantom read: A transaction that re-executes a query, returning a set of rows that satisfy a search condition and then finds that the set of rows that have been satisfying the condition has changed due to another recently-committed transaction.
  • Non-repeatable read: A transaction that re-reads the data that it has previously read and then finds that data has already been modified by another transaction (that committed since the initial read).
  • Dirty read : A transaction when reads data that is written by a concurrent uncommitted transaction is the dirty read.

It is a free open source GUI tool PostgreSQL database administration tool for Windows, Mac OS X, and Linux system. It is used for information retrieval, development, testing, and ongoing maintenance of Databases.

CTIDs is a field, which exists in every PostgreSQL table and is known to identify specific physical rows according to their block and offset positions within a particular table. They are used by index entries to point to physical rows. It is unique for each record in the table and easily denotes the location of a tuple. A logical row’s CTID changes when it is updated, so the CTID cannot be used as a long-term row identifier. However, it is sometimes useful to identify a row within a transaction when no competing update is expected.

Before you can have access to the database, you must be able to start the database server. The server program of the database is called Postgres. The Postgres program must know where to find the data it is supposed to use. This is done with the -D option. Thus, the simplest way to start the server is:

  1. /usr/local/etc/rc.d/ start
  2. /usr/local/etc/rc.d/PostgreSQL start

Conclusion: PostgreSQL, sometimes known as Postgres, is a relational database management system (RDBMS) that focuses on flexibility and SQL conformance. It was initially known as POSTGRES, after the Ingres database built at the University of California, Berkeley. The project was renamed PostgreSQL in 1996 to reflect its SQL support. The development team opted to preserve the name PostgreSQL and the alias Postgres after a review in 2007.Transactions with Atomicity, Consistency, Isolation, and Durability (ACID) qualities, automatically updatable views, materialised views, triggers, foreign keys, and stored procedures are all available in PostgreSQL. It can manage a wide range of workloads, from single computers to data warehouses or Web services with a large number of concurrent users. It is also available for Windows, Linux, FreeBSD, and OpenBSD, and is the default database for macOS Server.

Multiversion concurrency control (MVCC) in PostgreSQL regulates concurrency by giving each transaction a “snapshot” of the database, allowing modifications to be made without affecting other transactions. This obviates the requirement for read locks and assures that the database follows ACID guidelines. Read Committed, Repeatable Read, and Serializable are the three levels of transaction isolation provided by PostgreSQL. Requesting a Read Uncommitted transaction isolation level instead offers read committed because PostgreSQL is immune to dirty reads. The serializable snapshot isolation (SSI) mechanism in PostgreSQL allows for full serializability.

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