Reynold Xin created SPARK-13136:
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Summary: Data exchange (shuffle, broadcast) should only be handled
by the exchange operator
Key: SPARK-13136
URL: https://issues.apache.org/jira/browse/SPARK-13136
Project: Spark
Issue Type: Improvement
Components: SQL
Reporter: Reynold Xin
In an ideal architecture, we have a very small number of physical operators
that handle data exchanges, and the rest simply declare the input data
distribution needed and let the planner inject the right Exchange operators.
We have almost that, except the following few operators:
1. Limit: does its own shuffle or collect to get data to a single partition.
2. Except: does its own shuffle; note that this operator is going away and will
be replaced by anti-join (SPARK-12660).
3. broadcast joins: broadcast joins do its own broadcast, which is a form of
data exchange.
Here are a straw man for limit. Split the current Limit operator into two: a
partition-local limit and a terminal limit. Partition-local limit is just a
normal unary operator. The terminal limit requires the input data distribution
to be a single partition, and then takes its own limit. We then update the
planner (strategies) to turn a logical limit into a partition local limit and a
terminal limit.
For broadcast join, it is more involved. We would need to design the interface
for the physical operators (e.g. we are no longer taking an iterator as input
on the probe side), and allow Exchange to handle data broadcast.
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