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).