[HIVE-21398] Columns which has estimated statistics should not be considered as unique keys

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 4.0.0-alpha-1
• Assignee: Zoltan Haindrich
• Reporter: Zoltan Haindrich
• Created: 06/Mar/19 14:49
• Updated: 17/Nov/22 08:49
• Resolved: 14/Mar/19 06:57

Description

Right now for a column to qualify as a unique column it has to meet the criteria:

NDV >= numRows

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".