Kimahriman opened a new pull request #32987:
URL: https://github.com/apache/spark/pull/32987
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### What changes were proposed in this pull request?
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I am proposing to add support for conditionally evaluated expressions during
subexpression elimination. Currently, only expressions that will definitely be
always at least twice are candidates for subexpression elimination. This PR
updates that logic so that expressions that are always evaluated at least once
and conditionally evaluated at least once are also candidates for subexpression
elimination. This helps optimize a common case during data normalization and
cleaning and want to null out values that don't match a certain pattern, where
you have something like:
```
transformed = F.regexp_replace(F.lower(F.trim('my_column')))
df.withColumn('normalized_value', F.when(F.length(transformed) > 0,
transformed))
```
or
```
df.withColumn('normalized_value', F.when(transformed.rlike(<some regex),
transformed))
```
In these cases, `transformed` will always be fully calculated twice, because
it might only be needed once. I am proposing creating a subexpression for
`transformed` in this case.
In practice I've seen a decrease in runtime and codegen size of 10-30% in
our production pipelines that heavily make use of this type of logic.
The only potential downside is creating extra subexpressions, and therefore
function calls, more than necessary. This should only be an issue for certain
edge cases where your conditional overwhelming evaluates to false. And then the
only overhead is running your conditional logic potentially in a separate
function rather than inlined in the codegen. I added a config to control this
behavior if that is actually a real concern to anyone, but I'd be happy to just
remove the config.
I also updated some of the existing logic for common expressions in coalesce
and when that are actually better handled by the new logic, since you are only
guaranteed to have the first value of a Coalesce evaluated, as well as the
first conditional of a CaseWhen expression.
### Why are the changes needed?
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To increase the performance of conditional expressions.
### Does this PR introduce _any_ user-facing change?
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No, just performance improvements.
### How was this patch tested?
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New and updated UT.
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