[HIVE-2146] Block Sampling should adjust number of reducers accordingly to make it useful

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 0.8.0
• Hadoop Flags: Reviewed
• Assignee: Siying Dong
• Reporter: Siying Dong
• Created: 03/May/11 01:26
• Updated: 16/Dec/11 23:56
• Resolved: 04/May/11 19:07

Description

Now number of reducers of block sampling is not modified, so that queries like:
select c from tab tablesample(1 percent) group by c;
can generate huge number of reducers although the input is sampled to be small.
We need to shrink number of reducers to make block sampling more useful.
Since now number of reducers are determined before get splits, the way to do it probably is not clean enough, but we can do a good guess.