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What is a correlated subquery, and why can it be a performance trap?

Correlated subqueries read naturally but can execute per row (O(n squared)), a classic slow-query cause. The signal is recognizing them and rewriting as joins or window functions.

Updated Aug 2026 · Grounded in real Applied AI Engineer interview loops and written to a senior-engineer editorial bar.

Correlated subqueries read naturally but can execute per row (O(n squared)), a classic slow-query cause. The signal is recognizing them and rewriting as joins or window functions.

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