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Subqueries vs CTEs: Query Optimizer Internals & Memory Spooling Explained

Many engineers believe Common Table Expressions (CTEs) are always faster than subqueries. In modern SQL Server (and PostgreSQL), that is a myth . Here is what actually happens under the hood: 1.

  • CTEs and subqueries treated similarly by SQL optimizer in recent versions
  • CTEs offer readability and pipeline stacking advantages
  • Reusing a CTE multiple times may cause performance overhead

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