optimizer: Correct the comment on zero counts in the subquery-size guard#37501
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optimizer: Correct the comment on zero counts in the subquery-size guard#37501ggevay wants to merge 1 commit into
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The previous comment said constant folding folds an empty subquery's count to a literal 0, but folding an MIR count over an empty collection yields an empty collection, not a 0. The actual mechanism (verified by tracing the original failure from MaterializeInc#37155 on the commit where it occurred): over a provably empty subquery body, EquivalencePropagation vacuously rewrites the guard's count(true) to count(null), ReduceElision then elides the Reduce into a literal 0 via on_unique, LiteralLifting substitutes that literal into the guard's argument, and FlatMapElimination eagerly evaluates table functions with all-literal arguments during optimization. Comment-only change. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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Comment-only change. Followup to #37155
The previous comment said constant folding folds an empty subquery's count to a literal 0, but folding an MIR count over an empty collection yields an empty collection, not a 0. The actual mechanism (verified by tracing the original failure from #37155 on the commit where it occurred): over a provably empty subquery body, EquivalencePropagation vacuously rewrites the guard's count(true) to count(null), ReduceElision then elides the Reduce into a literal 0 via on_unique, LiteralLifting substitutes that literal into the guard's argument, and FlatMapElimination eagerly evaluates table functions with all-literal arguments during optimization.