POSTGRESQL · EASY CODEX

PostgreSQL Interview Questions — Easy

Q: What is a primary key and how does Postgres implement it?

Uniquely identifies rows; PRIMARY KEY creates a unique B-tree index + NOT NULL implicitly. One PK per table; choice of int/bigserial vs UUID trade-offs (size, ordering, fragmentation). PK index backs

Q: What does SERIAL vs IDENTITY vs UUID mean for keys?

SERIAL legacy pseudo-type creating sequence default; GENERATED ALWAYS AS IDENTITY SQL-standard preferred (protects against manual id inserts). Sequences have gaps on rollback by design. UUIDv4 random

Core concepts, definitions, basic syntax, and first principles expected in round 1 screening.

25 Theory Questions5 Free Model Answers

THEORY QUESTIONS & SOLUTIONSShowing 5 of 25 questions

Q1What is PostgreSQL and what distinguishes it?

Advanced open-source object-relational DB known for standards compliance, extensibility (custom types/extensions), and correctness.
Standouts: rich indexing (B-tree/GIN/GiST/BRIN), full-text search, JSONB, window functions, MVCC without read locks, transactional DDL.
Single license (PostgreSQL License), no paid editions - feature parity everywhere.

Q2What is a primary key and how does Postgres implement it?

Uniquely identifies rows; PRIMARY KEY creates a unique B-tree index + NOT NULL implicitly.
One PK per table; choice of int/bigserial vs UUID trade-offs (size, ordering, fragmentation).
PK index backs fast lookups; clustered tables don't exist like SQL Server - heap storage with index-organized access via the index only.

Q3What are the main differences between CHAR, VARCHAR and TEXT?

CHAR(n): blank-padded fixed length (rarely desired).
VARCHAR(n): variable with limit enforced.
TEXT: unlimited - Postgres performance identical across all three; use TEXT + CHECK constraint for length when needed. Interview nugget: unlike other engines, there's NO performance penalty choosing TEXT.

Q4What does SERIAL vs IDENTITY vs UUID mean for keys?

SERIAL legacy pseudo-type creating sequence default; GENERATED ALWAYS AS IDENTITY SQL-standard preferred (protects against manual id inserts).
Sequences have gaps on rollback by design.
UUIDv4 random → index bloat/fragmentation on big tables; UUIDv7/time-ordered mitigates. Choose bigint identity for internal high-write tables.

Q5What is NULL and how do IS NULL / COALESCE work?

NULL = unknown/absent, not zero/empty string.
Comparisons yield NULL (WHERE col <> 'x' excludes NULLs!) - explicit IS NULL/IS NOT NULL required.
COALESCE(a,b,c) first non-null; NULLIF(a,b) returns null when equal (divide-by-zero guard pattern).

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