PERFORMANCE: Implement a map-side group operator to speed up processing of
ordered data
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Key: PIG-984
URL: https://issues.apache.org/jira/browse/PIG-984
Project: Pig
Issue Type: New Feature
Reporter: Richard Ding
The general group by operation in Pig needs both mappers and reducers (the
aggregation is done in reducers). This incurs disk writes/reads between
mappers and reducers.
However, in the cases where the input data has the following properties
1. The records with the same key are grouped together (such as the data is
sorted by the keys).
2. The records with the same key are in the same mapper input.
the group by operation can be performed in the mappers only and thus remove the
overhead of disk writes/reads.
Alan proposed adding a hint to the group by clause like this one:
{code}
A = load 'input' using SomeLoader(...);
B = group A by $0 using "mapside";
C = foreach B generate ...
{code}
The proposed addition of using "mapside" to group will be a mapside group
operator that collects all records for a given key into a buffer. When it sees
a key change it will emit the key and bag for records it had buffered. It will
assume that all keys for a given record are collected together and thus there
is not need to buffer across keys.
It is expected that "SomeLoader" will be implemented by data systems such as
Zebra to ensure the data emitted by the loader satisfies the above properties
(1) and (2).
It will be the responsibility of the user (or the loader) to guarantee these
properties (1) & (2) before invoking the mapside hint for the group by clause.
The Pig runtime can't check for the errors in the input data.
For the group by clauses with mapside hint, Pig Latin will only support group
by columns (including *), not group by expressions nor group all.
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