Hot key fanout should not distribute keys to all shards.
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core
java
P3
python
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Description
The goal is to reduce the number of value sent to a single post-GBK worker. If combiner lifting happens, each bundle will sends a single value per sub-key, causing an N-fold blowup in shuffle data and N reducers with the same amount of data to consume as the single reducer in the non-fanout case.
Imported from Jira [BEAM-4565](https://issues.apache.org/jira/browse/BEAM-4565). Original Jira may contain additional context.
Reported by: robertwb.
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