Hello,
We have a class which extends RelBuilder to add some extra functionality, but
we are having issues with aliases being erased when popping the builder stack,
and there isn’t much recourse since the Frame stack is all private.
For example, this is valid:
```
relBuilder.scan("T101").empty().field(1, "T101", "C0_7009")
```
Because the `RelBuilder.empty` implementation copies the alias in the Frame
from the top of the Frame stack before the `empty` call, and uses it as the
name in the frame created *by* the `empty` call.
But people extending RelBuilder (us) cannot access or modify those private
members.
So we have no way to replace the node at the top of the stack without
eliminating the alias, and making referencing that alias illegal.
For example, lets say we overrode the `empty()` method do effectively replace
the node that would be created by the base `empty()` implementation
```
@Override
public RelBuilder empty(){
super.empty();
// pop result from stack to maintain proper stack arrangement
RelNode oldEmptyNode = this.build();
// and make new node
RelNode newEmptyNode = createNewEmptyNode( oldEmptyNode );
// push new node on the stack
this.push(newEmptyNode);
}
```
By popping the stack, we erase the alias information, so calling `field` with
an alias after `empty`throws an exception, since after the empty call
(specifically the `build` call) the alias in the Frame no longer exists.
The only solution to this would be to call `.as` immediately after the `push`
call to restore the alias:
```
@Override
public RelBuilder empty(){
super.empty();
// get the current alias
String currentAlias = ???;
// pop result from stack to maintain proper stack arrangement
RelNode oldEmptyNode = this.build();
// and make new node
RelNode newEmptyNode = createNewEmptyNode( oldEmptyNode );
// push new node on the stack
this.push(newEmptyNode);
// re-use the alias that existed before popping the node
this.alias(currentAlias);
}
```
But there doesn’t seem to be an existing way to ask what the *current* alias
is, so that it can be used in the `.as` call.
As it stands, it appears that it is not possible to add extension to RelBuilder
which either (a) cannot rely on an existing method to set-up the Frame or (b)
would cause the frame to be popped.
Is that correct? Or is there actually a way to see what the top Frame’s alias
is without modifying the core RelBuilder code?
Thanks!
-Ian Bertolacci
