Optimize CASE expressions by removing WHEN false branches

[SidChaudhary7]

Which issue does this PR close?

• ClosesSimplify CASE WHEN false THEN ... #17590
• ClosesExpression Simplification forExpr::Case expressions #1693

Rationale for this change

This PR implements an optimization for CASE expressions that removes unreachable WHEN false branches and handles WHEN true cases that make subsequent branches unreachable.

For example:

CASE WHEN false THEN 1-- never executed
ELSE 2 END

Are there any user-facing changes?

No

This is my first contribution to DataFusion. Could a committer please trigger the CI tasks?
@alamb@andygrove@tustvold@ursabot

[alamb]

This is my first contribution to DataFusion. Could a committer please trigger the CI tasks? @alamb@andygrove@tustvold@ursabot

Done!
Thanks for the PR @SidChaudhary7

Can you also put a note in the PR description about what issue this fixes?

[Jefffrey]

I suggest looking at this recent PR for a good reference on how to implement this: #17602

[Jefffrey]

Does this if guard make the below case arm unreachable?

[alamb]

Yes, I think so

In general I think we could combine this branch with the one above (that checks for a single true clause) 🤔

So the logic would be

• if the WHEN exprs have any true or false literals
• then rewrite the when_then expr, removing all false literals and any arms after the first true

[alamb]

Thank you for this PR @SidChaudhary7

It seems there are a few failing tests -- can you look into them?

[alamb]

Once we merge this pr from @jackkleeman

• Add case expr simplifiers for literal comparisons #17743
  I think we can adjust this PR and get it in

If @SidChaudhary7 doesn't have time by tomorrow, I'll try and push it along

[petern48]

I don't mind taking it if you'd rather save your time for other things. Should be pretty straightforward for me since I did the previous WHEN TRUE PR. I'll leave that up to you @alamb

[alamb]

I don't mind taking it if you'd rather save your time for other things. Should be pretty straightforward for me since I did the previous WHEN TRUE PR. I'll leave that up to you @alamb

That would be great @petern48 -- thank you for the offer! Please just ping me on the new PR 🙏