explain: improve projection explain about risinglight HOT 2 OPEN

The reason for the repeat is that as is eliminated at binder stage and doesn't exist in the plan. In egg-based planner, common expressions always share the same node. So I didn't introduce the as node in order not to make things complicated.

I think there are 2 possible solutions for this issue:

1. Resolve common expressions from child nodes and assign temporary variables for them in EXPLAIN. The output would be like:

   Projection { exprs: [ $0 ] }
   └── Projection { exprs: [ + { lhs: v1, rhs: v1 } as $0 ] }
       ...
   

2. Keep as variable information by introducing as node to the planner. However, this may break some optimization rules and lead to unknown behavior.

from risinglight.

The reason for the repeat is that as is eliminated at binder stage and doesn't exist in the plan. In egg-based planner, common expressions always share the same node. So I didn't introduce the as node in order not to make things complicated.

I think there are 2 possible solutions for this issue:

1. Resolve common expressions from child nodes and assign temporary variables for them in EXPLAIN. The output would be like:

   Projection { exprs: [ $0 ] }
   └── Projection { exprs: [ + { lhs: v1, rhs: v1 } as $0 ] }
       ...
   

2. Keep as variable information by introducing as node to the planner. However, this may break some optimization rules and lead to unknown behavior.

It looks like option 1 might be simpler

from risinglight.