You have a query using columns customer_id and order_date. The index is created only on customer_id. Why might this cause a performance issue?

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SQL - Indexes and Query Performance

You have a query using columns customer_id and order_date. The index is created only on customer_id. Why might this cause a performance issue?

ABecause indexes slow down all queries

BBecause customer_id is not a valid column

CBecause the index does not cover all columns used, causing table lookups

DBecause order_date cannot be indexed

Step-by-Step Solution

Solution:

Step 1: Identify columns in the index
The index covers only customer_id, not order_date.
Step 2: Understand query needs
The query uses both columns, so the index alone cannot satisfy it fully, causing extra table lookups.
Final Answer:
Because the index does not cover all columns used, causing table lookups -> Option C
Quick Check:
Partial index coverage = extra table access [OK]

Quick Trick: Index must cover all query columns to avoid table reads [OK]

Common Mistakes:

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