GH-44501: [C#] Use an index lookup for O(1) field index access

[vthemelis]

Fixes #44501

Rationale for this change

This should speed up the reasonably common operation of looking up a column via name in Schema or RecordBatch.

I would argue that the fact that this operation was previously O(N) (now O(1)) is unintuitive and could easily lead to accidental performance woes.

Note that I would like to also replace the existing Lookups with signature string -> Field but unfortunately those use StringComparer.Default instead of StringComparer.CurrentCulture (which I'm using to make the changed public function backwards compatible). Not sure if this is intentional.

What changes are included in this PR?

This PR simply memoizes the index lookup of the fields. It should not break any existing behaviour.

It also fixes a recent regression about the behaviour of the changed function in the case of using a custom comparator and have no matches in the schema.

Are these changes tested?

All code paths changed are covered by new unit tests.

Are there any user-facing changes?

Yes, the lookup function throws an exception when there are no matches fuels rather than returning -1. This was a regression introduces O(days) ago.

• GitHub Issue: [C#] Column(string) method in RecordBatch is linear to the number of columns #44501

[github-actions]

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[github-actions]

⚠️ GitHub issue #44501 has been automatically assigned in GitHub to PR creator.

[vthemelis]

@CurtHagenlocher it looks like my tests are failing in CI but I can't replicate this locally.

[vthemelis]

Oh, I see the issue. The CI does a merge into main first and this has changed since my PR: #44576

The PR above has changed the lookup behaviour when the field is not matched.

CC @georgevanburgh

[vthemelis]

@CurtHagenlocher and @georgevanburgh I've reverted to the original behaviour (throwing an InvalidOperationException) for now. Let me know if you prefer keeping the return -1 regression.

[CurtHagenlocher]

I'm a bit tied up with work but will look at this by Friday.

[vthemelis]

I'm a bit tied up with work but will look at this by Friday.

Thank you very much!

[CurtHagenlocher]

Please don't make unnecessary stylistic changes.

[vthemelis]

Sure, will undo this.

[CurtHagenlocher]

Thanks for the change! (and sorry for the delay) I left a few suggestions.

[CurtHagenlocher]

This is a breaking change. Is there a specific reason for it?

[vthemelis]

This is actually fixing a regression that was added in #44576

The function would previously throw InvalidOperationException due to .First(...). The description of the PR is erroneous in asserting that -1 is returned in the case of no match as the exception would first be thrown by the argument of the .IndexOf(...) function.

I noticed this because I wrote my unit tests before that PR was merged. Are you happy with keeping the return -1 regression?

[CurtHagenlocher]

I think for new code, we should use Ordinal. I don't think CurrentCulture really makes sense here.

[vthemelis]

If I change to Ordinal then this would be a breaking change as this is used in GetFieldIndex(string). Are you okay with changing the behaviour?

[vthemelis]

Of course, I can then only use the lookup if the user has provided Ordinal as the comparator but this would defeat the purpose of this PR which is to make lookup of Columns in RecordBatch O(1).

[vthemelis]

Or I could change this in a separate PR that implements your suggestions here #44650 (comment) ?

[CurtHagenlocher]

Can we also expose this in a way that lets users get all the indexes for a particular field name?

[vthemelis]

Sure, I'm happy to add this, though it feels like it's adding further to the scope of the PR. I'm happy to implement any API suggestions that you may have; what would be your preferred way?

[vthemelis]

Thanks for the change! (and sorry for the delay) I left a few suggestions.

Thank you very much for your review @CurtHagenlocher ! I added some replies to your comments. Let me know your thoughts and will proceed with changes.