Change Symbol to VariableReferenceExpression in PlanNode

[rongrong]

Step 1: Add outputVariables to all PlanNode while keeping outputSymbols. No logic change.
Step 2: Start using outputVariables instead of outputSymbols in planning / optimization.
Step 3: Remove outputSymbols.

This will take a while. I might or might not get it done eventually... -_-

[hellium01]

It is so great that some one finally are trying to tackle this problem :) No more typeProviders!!!

[hellium01]

This should be VariableReference as well?

[rongrong]

All Symbol in all PlanNode should be variables. They can't all happen in one diff.

[hellium01]

We don't need outputSymbols here. We can convert to List<Symbol> from List<VariableReferenceExpression> very easily.

[rongrong]

I tried that first. It means a lot of unnecessary code change. I think adding variable separate from symbol then remove symbol is cleaner.

[hellium01]

Given we need to do this conversion very frequently, add a utility function in symbolAllocator to get VariableReferenceExpression?

context.getSymbolAllocator().newSymbol(...);
context.getSymbolAllocator().getVariableReference(symbol);

[rongrong]

It doesn't really matter. However you write it now, eventually these will be changed again. None of these will be here once the refactor is done.

[hellium01]

We can just replace all internal Symbol to Variable? We can do a conversion whenever we can getOutput.

[rongrong]

If you can get that work feel free to do so.

[hellium01]

No need to have duplicated makerSymbol and markerVariable in constructor.

[hellium01]

distinctSymbols/hashSymbol should be VariableReference?

[rongrong]

One thing at a time. Otherwise each diff will be huge.

[rongrong]

Not ready for review, only for entertainment. ^_^