Senior LevelSQL & MySQL
What is a Loose Index Scan (Skip Scan) and how does it optimize GROUP BY and range queries?
❓ Interview Prompt: Explain Loose Index Scan (Skip Scan in MySQL 8.0), reading distinct B-Tree index prefixes without scanning all leaf nodes, and optimizing DISTINCT/GROUP BY.
A Loose Index Scan (Skip Scan) allows MySQL to skip reading duplicate index keys, executing $O(K)$ reads (where $K$ is unique key count) instead of $O(N)$ row scans:
How Skip Scan Works:
For a composite index '(category_id, price)':
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Query: 'SELECT DISTINCT category_id FROM products;'
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The engine reads the first 'category_id=1', jumps directly to 'category_id=2', skipping all intermediate price nodes!
Code Implementation Example
sql
-- MySQL 8.0 Skip Scan optimization in EXPLAIN output
EXPLAIN SELECT DISTINCT category_id FROM products;
-- Extra field displays: Using index for group-by (scanning skip index)💡Verifying Loose Index Scan optimization in EXPLAIN.
🎯 Interview Key Takeaway
Design composite indexes so MySQL can leverage Loose Index Scans to accelerate DISTINCT and GROUP BY queries.
#MySQL 8#Skip Scan#Loose Index Scan#Performance#Senior
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