datafusion doesn't process predicate pushdown correctly when there is outer join

[xudong963]

Describe the bug

Now datafusion doesn't process predicate pushdown correctly when there is outer join. I have discussed much in #1339 with @alamb. I think we ended up with a consensus.

Notes that filtering conditions fall into two categories: join condition and where condition, I'll explain them separately in the following:

1. join condition: if the filter condition contains the non-nullable-side table, it can't be pushed down.
2. where condition: if the filter condition contains the nullable-side table, it can't be pushed down. But it's can be optimized, I describe how to process the case in convert outer join to inner join to improve performance #1585.

Additional context
related issues and pr: #1321 , #1585, #1339

[xudong963]

cc @alamb

[james727]

I can try this one if no one is working on it yet.

[xudong963]

I can try this one if no one is working on it yet.

I believe no one is working on it, please do it! Thanks, @james727 If you have any problems, please feel free to ping me!