db-index-hygiene (M12)

The inverse of M11: M11 adds missing indexes, M12 removes the ones that only cost. Every index slows writes and consumes storage and cache; duplicates and never-read indexes are pure overhead. Feeds the Performance & Scale score (axis performance, relational Higiene de índices w16).

What it checks

1. Exact duplicates — two indexes with the identical key column list (and same type/predicate); one is redundant.
2. Redundant prefix — index on (a) is fully covered by an existing index on (a, b); the shorter one can usually be dropped (B-tree leftmost-prefix rule).
3. Unused — an index with no scans over a representative window (Tier-2 idx_scan = 0). This is runtime truth: never declared unused from static analysis alone.

Score / axis

Feeds performance only (relational Higiene de índices w16; folds into Indexación in NoSQL profiles).

Tier-0 (static)

Parse declared indexes and detect exact duplicates and leftmost-prefix redundancy from the DDL alone (established for duplicates — purely structural). Unused-index detection is runtime-only and emits status: needs_api at Tier-0 (never a silent pass) with a nudge to Tier-2.

Tier-1/2 (verification query, Postgres)

Duplicate/redundant (Tier-1 catalog):

SELECT indrelid::regclass AS tbl, array_agg(indexrelid::regclass) AS dupes
FROM pg_index GROUP BY indrelid, indkey, indclass, indpred
HAVING count(*) > 1;

Unused (Tier-2, requires sustained stats):

SELECT relname AS tbl, indexrelname AS idx, idx_scan, pg_size_pretty(pg_relation_size(indexrelid)) AS sz
FROM pg_stat_user_indexes WHERE idx_scan = 0 ORDER BY pg_relation_size(indexrelid) DESC;

Method index_check / query_stat. The Tier-2 result confirms M12.*.unused as established; without a representative window it stays directional, and if stats are needed to decide → needs_api.

Findings

Emit findings per schema/finding.schema.json. Examples:

• M12.users.duplicate_email_index — two indexes on the same key (severity:2, warn, axis performance, confidence established from DDL, fixable: proposed — DROP INDEX).
• M12.orders.redundant_prefix_index — (status) covered by (status, created_at) (severity:1, warn, established/directional, fixable: proposed).
• M12.events.unused_index — zero scans over the window (severity:2, warn, established Tier-2 / needs_api at Tier-0, fixable: proposed).

Each finding: evidence.observed quotes the index DDL or the catalog/stats row verbatim; verification.reproduce is the query above referencing $DATABASE_URL; expected_impact is banded + confidence-tagged (no naked %).

Honesty

• An unused index over a short window may be hit by month-end/quarterly jobs — always require a representative window and mark short-window verdicts directional, not established.
• A unique/constraint-backing index is not droppable even if "unused" for scans — it enforces integrity; never recommend dropping it.
• Drops are proposed (never auto): removal is not trivially reversible at scale and intent may be deliberate. Suggest DROP INDEX CONCURRENTLY in the fix preview.