[IMPALA-6024] Add minimum sample size for COMPUTE STATS TABLESAMPLE

• Type: Sub-task
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: Impala 2.10.0, Impala 2.11.0
• Fix Version/s: Impala 2.12.0
• Component/s: Frontend
• Target Version: Impala 2.12.0
• Epic Color: ghx-label-6
• Assignee: Alexander Behm
• Reporter: Alexander Behm
• Created: 06/Oct/17 20:43
• Updated: 31/Jan/18 19:29
• Resolved: 31/Jan/18 19:29

Description

We should introduce a minimum sample size in bytes for COMPUTE STATS TABLESAMPLE. Reasons:

• For small tables sampling does not make sense. Accurate stats can be obtained cheaply without sampling.
• Very small sample sizes mostly do not make sense - some minimum of data is required to get meaningful stats.

I think a 1GB minimum might be a good choice and ideally this minimum sample size would be configurable.

Many other DBMS have stats collection with sampling and in many cases a minimum sample size is required to get any meaningful stats.