[Java] Fix problems with current union comparison logic

[asfimport]

There are some problems with the current union comparison logic. For example:

1. For type check, we should not require fields to be equal. It is possible that two vectors' value ranges are equal but their fields are different.

Reporter: Liya Fan / @liyafan82
Assignee: Liya Fan / @liyafan82

PRs and other links:

• GitHub Pull Request #5544

_{Note: This issue was originally created as ARROW-6738. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Is this actually a good idea to include this in a bugfix release? It's a behavior change AFAICT.

[asfimport]

Liya Fan / @liyafan82:
@pitrou Thanks for your comments.
IMO, the only behavioral change is UnionVector#getVector, which is changed from private to public.

I hope this bug can be fixed in the next release, because our current code base does not compare union vectors correctly, and this problem, I belive, is kind of fundamantal.

[asfimport]

Wes McKinney / @wesm:
I do not think we should block 0.15.1 over this because there is no regression

[asfimport]

Liya Fan / @liyafan82:
@wesm Thanks for the comment.
I am not clear about the standard about which JIRA should be included in 0.15.1.

I just think this is a fundamental problem (comparison of Union vectors), so I want the patch to be relased ASAP.

If you think it should not be included in 0.15.1, please feel free to remove it from 0.15.1.

[asfimport]

Micah Kornfield / @emkornfield:
Issue resolved by pull request 5544
#5544