[IMPALA-4617] Remove duplication of isConstant() and IsConstant() in frontend and backend

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: Impala 2.8.0
• Fix Version/s: Impala 2.9.0
• Component/s: Backend
• Assignee: Tim Armstrong
• Reporter: Tim Armstrong
• Created: 07/Dec/16 21:56
• Updated: 31/Jan/17 18:09
• Resolved: 31/Jan/17 18:09

Description

Currently Expr.isConstant() and Expr::IsConstant() in the frontend and backend have duplicate logic to do exactly the same analysis. They need to be kept exactly in sync to avoid problems. We should plumb through the value of isConstant() from the frontend to avoid this duplication.

We need to be a little careful about how we do this in the frontend: Alex Behn mentioned that storing state in Exprs was risky, and also that naively calling isConstant() for each expr node was problematic since it could result in traversing the Expr tree many times.