An index stores key values in a secondary, sorted structure that allows the optimizer to locate the desired rows with logarithmic-time lookups instead of full table scans, greatly accelerating data retrieval while leaving the underlying table unchanged.

A. Indexes consume additional disk space; they do not assist with space allocation.

C. Because they add extra on-disk and in-memory structures (buffer cache pages), indexes increase, not reduce, memory and storage usage.

D. Query plans are regenerated by the optimizer each execution; an index may be chosen within a plan, but its creation does not itself “update” the plan.

1. PostgreSQL Documentation v15

Sec. 11.1 “Indexes and Index Types”: “The primary purpose of an index is to increase database performance … faster retrieval.”

2. Oracle Database Concepts 19c

Ch. 15 “Indexing and Partitioning

” p. 15-2: “Indexes provide faster access to data…”

3. Silberschatz

Korth & Sudarshan

“Database System Concepts

” 7th ed.

McGraw-Hill

§14.1

p. 481: “The basic motivation for creating an index is to speed up record retrieval.”

4. MySQL 8.0 Reference Manual

§8.3.1 “How MySQL Uses Indexes”: “Indexes are used to find rows with specific column values quickly.”