add brain

.brain/.agent/skills/database-optimization/mysql/references/covering-indexes.md

@@ -0,0 +1,47 @@
+---
+title: Covering Indexes
+description: Index-only scans
+tags: mysql, indexes, covering-index, query-optimization, explain
+---
+
+# Covering Indexes
+
+A covering index contains all columns a query needs — InnoDB satisfies it from the index alone (`Using index` in EXPLAIN Extra).
+
+```sql
+-- Query: SELECT user_id, status, total FROM orders WHERE user_id = 42
+-- Covering index (filter columns first, then included columns):
+CREATE INDEX idx_orders_cover ON orders (user_id, status, total);
+```
+
+## InnoDB Implicit Covering
+Because InnoDB secondary indexes store the primary key value with each index entry, `INDEX(status)` already covers `SELECT id FROM t WHERE status = ?` (where `id` is the PK).
+
+## ICP vs Covering Index
+- **ICP (`Using index condition`)**: engine filters at the index level before accessing table rows, but still requires table lookups.
+- **Covering index (`Using index`)**: query is satisfied entirely from the index, with no table lookups.
+
+## EXPLAIN Signals
+Look for `Using index` in the `Extra` column:
+
+```sql
+EXPLAIN SELECT user_id, status, total FROM orders WHERE user_id = 42;
+-- Extra: Using index ✓
+```
+
+If you see `Using index condition` instead, the index is helping but not covering — you may need to add selected columns to the index.
+
+## When to Use
+- High-frequency reads selecting few columns from wide tables.
+- Not worth it for: wide result sets (TEXT/BLOB), write-heavy tables, low-frequency queries.
+
+## Tradeoffs
+- **Write amplification**: every INSERT/UPDATE/DELETE must update all relevant indexes.
+- **Index size**: wide indexes consume more disk and buffer pool memory.
+- **Maintenance**: larger indexes take longer to rebuild during `ALTER TABLE`.
+
+## Guidelines
+- Add columns to existing indexes rather than creating new ones.
+- Order: filter columns first, then additional covered columns.
+- Verify `Using index` appears in EXPLAIN after adding the index.
+- **Pitfall**: `SELECT *` defeats covering indexes — select only the columns you need.