A table events has an index on event_type. You notice that queries filtering on event_type are slower than expected. The column has only three distinct values. What is the best explanation?

hard📝 Application Q9 of 15

SQL - Indexes and Query Performance

A table events has an index on event_type. You notice that queries filtering on event_type are slower than expected. The column has only three distinct values. What is the best explanation?

AThe table is empty, so indexes don't help

BLow cardinality columns with few distinct values make indexes less effective

CThe queries are using incorrect syntax

DThe index is corrupted and needs rebuilding

Step-by-Step Solution

Solution:

Step 1: Understand index effectiveness and cardinality
Indexes work best on columns with many unique values; low cardinality columns (few distinct values) often cause full scans instead.
Step 2: Evaluate other options
Corruption or syntax errors would cause errors, not just slow queries; empty tables don't cause slow queries.
Final Answer:
Low cardinality columns with few distinct values make indexes less effective -> Option B
Quick Check:
Low cardinality hurts index usefulness = D [OK]

Quick Trick: Indexes on low-cardinality columns often don't help [OK]

Common Mistakes:

Assuming all indexes speed queries equally
Blaming syntax for slow queries
Ignoring cardinality impact

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More SQL Quizzes

A table events has an index on event_type. You notice that queries filtering on event_type are slower than expected. The column has only three distinct values. What is the best explanation?

Step 1: Understand index effectiveness and cardinality

Step 2: Evaluate other options

Final Answer:

Quick Check:

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