[HIVE-2621] Allow multiple group bys with the same input data and spray keys to be run on the same reducer.

• Type: New Feature
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 0.9.0
• Assignee: Kevin Wilfong
• Reporter: Kevin Wilfong
• Created: 02/Dec/11 02:13
• Updated: 23/Apr/14 21:08
• Resolved: 03/Jan/12 18:10

Description

Currently, when a user runs a query, such as a multi-insert, where each insertion subclause consists of a simple query followed by a group by, the group bys for each clause are run on a separate reducer. This requires writing the data for each group by clause to an intermediate file, and then reading it back. This uses a significant amount of the total CPU consumed by the query for an otherwise simple query.

If the subclauses are grouped by their distinct expressions and group by keys, with all of the group by expressions for a group of subclauses run on a single reducer, this would reduce the amount of reading/writing to intermediate files for some queries.

To do this, for each group of subclauses, in the mapper we would execute a the filters for each subclause 'or'd together (provided each subclause has a filter) followed by a reduce sink. In the reducer, the child operators would be each subclauses filter followed by the group by and any subsequent operations.

Note that this would require turning off map aggregation, so we would need to make using this type of plan configurable.

Issue Links

• relates to: HIVE-2056 Generate single MR job for multi groupby query if hive.multigroupby.singlemr is enabled.