# deep-query-optimizer Multi-pass review-and-fix optimizer for SQL: dialect detection, sargability, index design, join/N+1 analysis, and EXPLAIN-verified rewrites that back out any regression. The SQL member of the family. Detects the dialect (Postgres, MySQL, SQLite, SQL Server), audits sargability, index design, joins and N+1 patterns, predicate logic, projection, pagination, subqueries/CTEs, and the EXPLAIN plan itself, then applies every Medium+ rewrite in place. When a database connection exists, verification is empirical: `EXPLAIN` / `EXPLAIN ANALYZE` must show the plan improved *and* the result set unchanged — a rewrite that fails either test is backed out. It also recommends index DDL where the plan shows the query is paying for a missing index. **Use it for:** "optimize this SQL", "run dqo", "tune this query", "why is this query slow", index-design review on a hot path. **Not for:** MongoDB MQL/aggregation (mongodb-expert's query optimizer spoke) · application-code N+1 fixes beyond the query itself ([code-deep-optimizer](/skills/code-deep-optimizer/)).