Took me a minute to even parse what 'constant folding' meant in this context.
Start by clarifying the query tree structure and the definition of 'safe' nodes for folding. Then outline a recursive post-order traversal that processes subqueries first, folds constant expressions in projections and predicates, and returns a simplified tree. Emphasize correctness, safety conditions, and handling of nested subqueries.
Pro tip: Mention that constant folding must respect SQL semantics like NULL propagation and short-circuit evaluation, and that you would add a configuration flag to enable/disable folding for debugging or A/B testing.
Ask about the exact node types, what constitutes a 'safe' fold (e.g., deterministic, no side effects), and how subqueries are represented. Confirm the expected output format.
Use post-order traversal to process children first, ensuring subqueries are simplified before parent nodes. Define a function that returns a simplified node and a flag indicating if any change occurred.
For each node, evaluate constant expressions in projections and predicates, replacing them with literal values. Handle logical operators with short-circuiting and NULL semantics.
Recursively convert nested queries into subqueries, applying the same folding process. Ensure that correlated subqueries are not incorrectly folded.
Return the new tree, and discuss performance gains, potential risks (e.g., changing semantics), and how to test the transformation.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.