feat(stats): Set distinct_count to Exact(1) when filter narrows column to single value#21077
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asolimando wants to merge 1 commit intoapache:mainfrom
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feat(stats): Set distinct_count to Exact(1) when filter narrows column to single value#21077asolimando wants to merge 1 commit intoapache:mainfrom
asolimando wants to merge 1 commit intoapache:mainfrom
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When a filter predicate collapses a column's interval to a single value (e.g. d_qoy = 1), the output has exactly 1 distinct value. Previously the original Parquet NDV was propagated, inflating GROUP BY output estimates for CTE self-join patterns like Q31.
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cc: @jonathanc-n |
adriangb
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Mar 20, 2026
| @@ -59,7 +59,7 @@ query TT | |||
| EXPLAIN SELECT * FROM test_table WHERE column1 = 1; | |||
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Can we add some tests with e.g. floating point numbers, strings? I think it should work just the same but....
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Which issue does this PR close?
Related: #20789 (uses NDV for equality filter selectivity, complementary - this PR improves the NDV output stats, that PR consumes them)
Rationale for this change
When a filter predicate collapses a column interval to a single value (e.g.
d_qoy = 1), the output column can only have one distinct value. Currentlydistinct_countis always demoted toInexact, losing this information.This matters for downstream optimizers that rely on
distinct_count, such as join cardinality estimation inestimate_inner_join_cardinality.What changes are included in this PR?
In
collect_new_statistics(filter.rs), when the post-filter interval haslower == upper(both non-null), setdistinct_counttoPrecision::Exact(1)instead of demoting the input NDV toInexact.Are these changes tested?
Yes, 4 unit tests:
a = 42) -> NDV becomesExact(1)a = 42 OR a = 22) -> interval does not collapse, NDV staysInexacta = 42 AND b > 10 AND c = 7) ->aandcgetExact(1),bstaysInexacta = 42, no min/max) -> interval analysis still resolves toExact(1)Are there any user-facing changes?
No breaking changes. Statistics consumers will now see
Exact(1)fordistinct_counton columns constrained to a single value by filter predicates.Disclaimer: I used AI to assist in the code generation, I have manually reviewed the output and it matches my intention and understanding.