Use column order that matches real filter patterns.
Section: Core concepts
Composite index left-prefix rule
sql
sql
INDEX (status, created_at, user_id)
Supports well:
WHERE status = ?
WHERE status = ? AND created_at >= ?
Usually not ideal alone for:
WHERE created_at >= ?Explanation
For many B-tree implementations, the leading columns matter most for efficient lookups.
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What an index does
Indexes trade storage and write cost for faster reads.
Prefer high-selectivity columns
Columns with many distinct values tend to benefit more.
Remember the write cost
Every extra index must be maintained during inserts, updates, and deletes.