What is a composite index and why does column order matter?

SQL indexes, transactions and performance · Freshers and experienced

Model answer

An index on several columns. Order matters because it can be used for queries filtering on the leftmost columns: an index on (country, city) helps WHERE country = 'IN' and WHERE country = 'IN' AND city = 'Pune', but not city alone. Put equality columns first, then range or sort columns.

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Indexes and Faster Queries: EXPLAIN for Beginners in the course SQL for Beginners: Learn Databases Step by Step.

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