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Feb 5, 2021 · Databases

Mastering Subquery Unnesting: Theory, Apply Operator, and Practical Rules

This article explains why correlated subqueries are a performance bottleneck, introduces the Apply (Correlated Join) operator, and presents a comprehensive set of transformation rules—including basic unnesting, Project/Filter push‑down, Aggregate handling, and Set‑operation rewrites—to convert subqueries into efficient join plans for modern SQL engines.