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## Which issue does this PR close? - Part of #20766 ## Rationale for this change After a filter reduces a table from 100 to 10 rows, or a LIMIT 10 caps the output, the NDV (e.g. 80) should not exceed the new row count. Without capping, join cardinality estimation uses an inflated denominator, leading to inaccurate estimates. ## What changes are included in this PR? Cap `distinct_count` at `num_rows` in three places to prevent NDV from exceeding the actual row count: - `max_distinct_count` in join cardinality estimation (`joins/utils.rs`) - `collect_new_statistics` in filter output statistics (`filter.rs`) - `Statistics::with_fetch` (`stats.rs`), which covers `GlobalLimitExec`, `LocalLimitExec`, `SortExec` (with fetch), `CoalescePartitionsExec` (with fetch), and `CoalesceBatchesExec` (with fetch) Note: NDV capping for `AggregateExec` is covered separately in #20926. ## Are these changes tested? - `test_filter_statistics_ndv_capped_at_row_count` - verifies NDV capped at filtered row count - 2 new join cardinality test cases - NDV > rows on both/one side - Updated `test_join_cardinality` expected values for capped NDV - `test_with_fetch_caps_ndv_at_row_count` - verifies NDV capped after LIMIT - `test_with_fetch_ndv_below_row_count_unchanged` - verifies NDV untouched when already below row count - All existing `with_fetch` tests pass ## Are there any user-facing changes? No public API changes. Only internal statistics estimation is affected. Disclaimer: I used AI to assist in the code generation, I have manually reviewed the output and it matches my intention and understanding.
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