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role-database:query-optimization

Cross-engine query optimization guide. EXPLAIN/EXPLAIN ANALYZE across PostgreSQL, MySQL, MongoDB, Cassandra, ClickHouse. Index strategies (B-tree, hash, GIN, GiST, BRIN, partial, covering, composite). N+1 detection, cursor-based pagination, materialized views, query plan analysis, slow query diag...

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SKILL.md

Full skill instructions

Query Optimization

Reference Files

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references/​explain-indexes.md

EXPLAIN analysis for PostgreSQL (red flags, buffer hit ratio), MySQL (type column, FORMAT=TREE), MongoDB (executionStats, COLLSCAN vs IXSCAN), ClickHouse (system.query_log). Index type comparison table (B-tree, Hash, GIN, GiST, BRIN, partial, covering). Composite index design rules (equality first, range last, selectivity order). Partial indexes and covering indexes with INCLUDE. Statistics and cardinality management. Load when: analyzing query plans, designing indexes, or fixing stale statistics.

references/​pagination-antipatterns.md

N+1 detection via pg_stat_statements, elimination patterns (eager load, DataLoader, window functions). Offset vs cursor-based pagination with multi-column cursor example. Materialized views: creation, blocking vs concurrent refresh, automated refresh strategies. Query anti-patterns: SELECT *, functions on indexed columns, correlated subqueries, implicit type conversions, missing LIMIT. Load when: fixing N+1 problems, implementing pagination, or eliminating slow query patterns.