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.agent/skills/database-design/indexing.md

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+# Indexing Principles
+
+> When and how to create indexes effectively.
+
+## When to Create Indexes
+
+```
+Index these:
+├── Columns in WHERE clauses
+├── Columns in JOIN conditions
+├── Columns in ORDER BY
+├── Foreign key columns
+└── Unique constraints
+
+Don't over-index:
+├── Write-heavy tables (slower inserts)
+├── Low-cardinality columns
+├── Columns rarely queried
+```
+
+## Index Type Selection
+
+| Type | Use For |
+|------|---------|
+| **B-tree** | General purpose, equality & range |
+| **Hash** | Equality only, faster |
+| **GIN** | JSONB, arrays, full-text |
+| **GiST** | Geometric, range types |
+| **HNSW/IVFFlat** | Vector similarity (pgvector) |
+
+## Composite Index Principles
+
+```
+Order matters for composite indexes:
+├── Equality columns first
+├── Range columns last
+├── Most selective first
+└── Match query pattern
+```