Avoid div-by-zero in row count estimations for empty tables #1992
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Forward-port of an 0.11.1 hotfix for the BitCraft alpha. What was happening was, `num_distinct_values` on an empty table returned 0, which `index_row_est` then used as a divisor, leading to a panic. It seems reasonably clear to me that the correct estimate of distinct values in an empty index is 0, not bottom, so adjusting `num_distinct_values` to return `Option<NonZeroU64>`, and then making `index_row_est` return 0 in that case, seems an obvious fix.
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Description of Changes
Forward-port of #1991 , an 0.11.1 hotfix for the BitCraft alpha, into master.
What was happening was,
num_distinct_valueson an empty table returned 0, whichindex_row_estthen used as a divisor, leading to a panic. It seems reasonably clear to me thatthe correct estimate of distinct values in an empty index is 0, not bottom, so adjusting
num_distinct_valuesto returnOption<NonZeroU64>, and then makingindex_row_estreturn 0 in that case, seems an obvious fix.API and ABI breaking changes
None
Expected complexity level and risk
1
Testing
Describe any testing you've done, and any testing you'd like your reviewers to do,
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