Zoltan Haindrich created HIVE-21398:
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Summary: Columns which has estimated statistics should not be
considered as unique keys
Key: HIVE-21398
URL: https://issues.apache.org/jira/browse/HIVE-21398
Project: Hive
Issue Type: Bug
Reporter: Zoltan Haindrich
Assignee: Zoltan Haindrich
Right now for a column to qualify as a unique column it has to meet the
criteria:
{code}
NDV >= numRows
{code}
when numRows is 1 this tends to be true ; but numRows is also 1 in cases when
we are kinda operate in the blind - don't know how many row there are - more
generatlly: with estimated column statistics.
As a sideeffect of qualifying all columns to be unique; after a few joins all
column combinations became unique....so for a join between 3 tables which have
(i,j,k) columns; then it will allocate {{i*j*k}} triplets of "unique column
triplets".
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