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GATE CSE 2026 Session 2 Q46: Dense vs sparse: hash index and B+ tree index

Question

An index in a DBMS is said to be dense if an index entry appears for every search-key value in the indexed file. Otherwise it is called a sparse index. Consider the following two statements.

S1: A hash index must be a dense index

S2: A B+B^+ tree index can be a sparse index

Which one of the following options is correct?

Try it: multiple choice: exactly one correct option

Select one option
Wrong answer costs 2/3 mark in GATE.
Show answer and explanation

Answer: A (official GATE 2026 final answer key)

Concept used: A sparse index relies on the file being sorted on the key, so only ordered indexes can be sparse.

First step: A hash lookup cannot land "near" a key and scan forward; a clustered B+ tree can point to blocks.

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Source: official GATE 2026 Session 2 CS question paper and final answer key, organised by IIT Guwahati. Question paper · Answer key

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