[Solved] A table sales has indexes on region and sale_date. You run this query:SELECT * FROM sales WHERE region = 'North' AND sale_date < '2023-01-01';The query is slow. What is a likely cause? | SQL

SQL - Indexes and Query Performance

A table sales has indexes on region and sale_date. You run this query:

SELECT * FROM sales WHERE region = 'North' AND sale_date < '2023-01-01';

The query is slow. What is a likely cause?

AThe table has no rows

BThe indexes are corrupted

CThe query optimizer is not using a combined index

DThe query syntax is invalid

Step-by-Step Solution

Solution:

Step 1: Understand index usage for multiple conditions
Separate indexes on region and sale_date may not be combined efficiently; a composite index on both columns would help.
Step 2: Check query and data
The syntax is valid and table likely has data; slow query suggests optimizer can't use indexes well.
Final Answer:
The query optimizer is not using a combined index -> Option C
Quick Check:
Composite indexes help multi-column filters = D [OK]

Quick Trick: Use composite indexes for multi-column WHERE filters [OK]

Common Mistakes:

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