[CALCITE-844] Push Project through Window

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 1.5.0
• Assignee: Julian Hyde
• Reporter: Sean Hsuan-Yi Chu
• Created: 14/Aug/15 00:53
• Updated: 10/Nov/15 08:02
• Resolved: 04/Sep/15 07:48

Description

The lack of Project under LogicalWindow hurts the performance.

Firstly of all, this issue happens when HepPlanner is used with ProjectToWindowRule.PROJECT rule.

A simple query like:

select sum(deptno) over(partition by deptno) as sum1 
from emp

produces

LogicalProject($0=[$9])
  LogicalWindow(window#0=[window(partition {7} order by [] range between UNBOUNDED PRECEDING and UNBOUNDED FOLLOWING aggs [SUM($7)])])
    LogicalTableScan(table=[[CATALOG, SALES, EMP]])

However, from performance standpoint, it is better to have a project between LogicalWindow and LogicalTableScan since only one column is used. Interestingly, when there is an expression in the window function. For example,

select sum(deptno + 1) over(partition by deptno) as sum1 
from emp"

produces

LogicalProject($0=[$2])
  LogicalWindow(window#0=[window(partition {0} order by [] range between UNBOUNDED PRECEDING and UNBOUNDED FOLLOWING aggs [SUM($1)])])
    LogicalProject(DEPTNO=[$7], $1=[+($7, 1)])
      LogicalTableScan(table=[[CATALOG, SALES, EMP]])

The LogicalProject below window can trim out useless columns or even be pushed into Scan, which is very important optimization Calcite can exploit.