Proposal: Rename MergeExec

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
The name MergeExec has long confused me.

The name MergeExec implies to me that its purpose is to "merge sorted streams" into another sorted stream

However, MergeExec does something more like UNION ALL of streams into a single stream, combining the input streams in some arbitrary output order. Something that looks to me like an actual "merge" operation is described in #362.

Describe the solution you'd like
I propose renaming MergeExec to UnionToSingleStreamExec to make it clearer

It could also potentially combined with the existing UnionExec (which preserves partitioning) as another flag

Describe alternatives you've considered
As this would be a fairly invasive (and backwards incompatible change) leaving the current naming is also a possibility.

[alamb]

Do you have any thoughts on this @Dandandan @jorgecarleitao @andygrove @returnString ?

[jorgecarleitao]

agree 👍

[andygrove]

I agree that we should change the name. If I had to pick a new name I would probably be thinking more along the lines of CoalescePartitionsExec since this is a specialized form of repartitioning. That said, I don't have a strong opinion on the new name.

[Dandandan]

I agree that the name is confusing. Union would be confusing too, as a UNION ALL doesn't actually combine the partitions into one and has 2 or more children instead of only 1.

CoalescePartitionsExec sounds great to me 👍

[Dandandan]

Another thing which makes some sense to me is making this a special case of RepartitionExec with a SinglePartition option (next to round robin / hash)

[alamb]

I likeCoalescePartitionsExec 👍