feat: Add selectivity metric to NestedLoopJoinExec for EXPLAIN ANALYZE #18481
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petern48
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Nov 4, 2025
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| // Selectivity Metric: Update total possibilities for the batch (left_rows * right_rows) | ||
| if let (Ok(left_data), Some(right_batch)) = | ||
| (self.get_left_data(), self.current_right_batch.as_ref()) | ||
| { | ||
| let left_rows = left_data.batch().num_rows(); | ||
| let right_rows = right_batch.num_rows(); | ||
| self.metrics.selectivity.add_total(left_rows * right_rows); | ||
| } |
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I originally tried to get the values and calculate num_left * num_right at the end (in a drop() method, similar to how it was done here for BuildProbeJoinMetrics. But found it difficult to get the correctness consistently right since these are all multiple streams.
Instead, I'm doing it incrementally. The basic idea here is that for each right batch, we add_total() the size of the full left buffer times the num_rows in the current batch. Doing that num_batches times gives us the equivalent of num_right * num_left.
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| self.metrics.join_metrics.output_batches.add(1); | ||
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| // Update output rows for selectivity metric | ||
| let output_rows = batch.num_rows(); | ||
| self.metrics.selectivity.add_part(output_rows); |
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Update output_rows at flush time.
Co-authored-by: Yongting You <[email protected]>
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Which issue does this PR close?
selectivitymetrics (forExplain Analyze) in Nested Loop Join #18407Rationale for this change
This new metric will give the user better visibility to see what portion of the possibilities is actually being matched.
What changes are included in this PR?
Add selectivity metric to NestedLoopJoinExec for EXPLAIN ANALYZE
Are these changes tested?
Added tests
Are there any user-facing changes?
Yes, new metric in explain analyze