# Indexing — Derived Rules

> Derives from `domains/data/first-principles.md` P5 (Indexing with Intent), P10 (Performance Awareness).

## Index for Queries, Not Tables (P5)

- An index serves a query. No query, no index.
- The query plan is the spec. `EXPLAIN` is the test. An index that is not used is dead weight.
- Index the columns you filter on (`WHERE`), join on (`JOIN`), and sort on (`ORDER BY`).

## Composite Indexes (P5, P10)

- Order matters: `INDEX(a, b)` serves `WHERE a = ? AND b = ?` and `WHERE a = ?`, but NOT `WHERE b = ?`.
- Put the most selective column first. Or the column used in every query. Depends on the workload.
- An index on every column is not a strategy. It is write amplification.

## Unique Indexes (P3 Invariants in Schema)

- A uniqueness constraint is a unique index. Use it for invariants: `email`, `username`.
- Unique indexes enforce; application checks defend. Both belong.
- A partial unique index: `UNIQUE(email) WHERE deleted_at IS NULL` — allows soft-deleted duplicates.

## Covering Indexes (P10)

- An index that covers all columns of a query is an "index-only scan" — no table lookup.
- PostgreSQL: `INCLUDE` clause. MySQL: all columns in the index.
- Use for hot queries. Don't cover everything; index size matters.

## Don't Over-Index (P10, core C8 Economy)

- Every index costs a write. The write budget is the index count.
- Indexes take disk and memory. A 1GB index on a 500MB table is a smell.
- Remove unused indexes. `pg_stat_user_indexes` shows usage. An unused index is debt.

## Migration and Indexes (P4 Migration Safety)

- Adding an index on a large table is expensive. Do it concurrently (`CREATE INDEX CONCURRENTLY`).
- An index migration that locks the table blocks writes. Plan for it.
- Build the index, then deploy the query that uses it. Not the reverse.

## What Violates Indexing Discipline

| Violation | Principle |
|-----------|-----------|
| Index on every column | P10, C8 Economy |
| No index on a foreign key | P10 (join performance) |
| `WHERE b = ?` with only `INDEX(a, b)` | P5 (wrong order) |
| Index created without checking the query plan | P5 (no intent) |
| `CREATE INDEX` (non-concurrent) on a 10M-row table in prod | P4 Migration Safety |