Devyn Cairns created SPARK-29881:
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Summary: Introduce API for manually breaking up dataset plan
Key: SPARK-29881
URL: https://issues.apache.org/jira/browse/SPARK-29881
Project: Spark
Issue Type: Wish
Components: SQL
Affects Versions: 2.4.4
Reporter: Devyn Cairns
I have an interesting situation where I'm calling functions that are relatively
expensive from Spark SQL, and then using the result several times in a loop
through {{transform}}.
Although the WholeStageCodegen is usually helpful, it always calls expressions
as they're used, which means that in the case of, for example:
{{SELECT transform(sequence(0, 32), x -> expensive_result * x)}}
{{FROM (}}
{{ SELECT expensive_operation(foo) AS expensive_result FROM source}}
{{)}}
the expensive_operation function will almost certainly be called 32 times for
each source row, without any explicit way to cache that value intermediately.
I've found a workaround for now is to insert something like {{.filter \{ _ =>
true }}} in the middle, which will create a barrier to whole-stage codegen
without much negative impact, aside from preventing other optimizations like
PushDown. This does indeed produce the intended result and expensive_operation
is only run once.
But it would be great to have an API on Dataset like {{.barrier()}} to
introduce an explicit barrier to whole-stage codegen without adding any
additional behavior or getting in the way of any PushDown optimizations.
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