Core purpose of SQL Server indexes:\nEvaluate the statement:\n\n"SQL Server indexes are special data structures used to improve performance."

Difficulty: Easy

Correct Answer: Correct

Explanation:


Introduction / Context:
Indexes in SQL Server organize data to accelerate lookups, joins, sorts, and predicates, much like a book index. They can dramatically reduce I/O and CPU for read-heavy workloads and can also improve write performance in some patterns by supporting seek access and narrowing locks. Understanding their role is foundational in database tuning.


Given Data / Assumptions:

  • We refer to the general case: clustered and nonclustered B-tree indexes as primary structures.
  • Specialized indexes (columnstore, XML, spatial, full-text) exist but share the performance goal.
  • Appropriate statistics and cardinality estimation are available.


Concept / Approach:
Indexes provide ordered access paths and mapping from key values to data locations. They reduce scans by enabling seeks, support efficient JOIN/ORDER BY/GROUP BY, and help the optimizer choose better plans. Trade-offs include additional storage, maintenance overhead on data modification, and potential write amplification.


Step-by-Step Solution:

Create a relevant index (e.g., CREATE INDEX ix ON dbo.Sales(CustomerId, OrderDate)).Re-execute queries: observe plan changes from scans to seeks.Measure I/O and CPU reductions via execution plans and SET STATISTICS IO/TIME.Validate that DML costs (INSERT/UPDATE/DELETE) are acceptable.


Verification / Alternative check:
Compare execution plans with and without indexes; use Query Store to confirm performance deltas; validate with realistic workloads.


Why Other Options Are Wrong:

  • Incorrect: Contradicts the core design of indexing.
  • SELECT-only: Indexes also help JOIN, GROUP BY, window functions, and sometimes constraints.
  • SSD-only / row-count-only: Benefits are logical and apply regardless of media or table size (though magnitude varies).


Common Pitfalls:
Over-indexing; redundant indexes; missing covering columns; stale statistics; ignoring write overhead.


Final Answer:
Correct

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