[CALCITE-808] Optimize ProjectMergeRule

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 1.4.0-incubating
• Assignee: Julian Hyde
• Reporter: Julian Hyde
• Created: 21/Jul/15 18:12
• Updated: 01/Sep/15 03:02
• Resolved: 21/Jul/15 22:02

Description

Optimize ProjectMergeRule to use RelOptUtil.pushPastProject rather than creating two RexPrograms and merging them. The current approach generates a lot of gc pressure.

One side effect will be to remove some identity projects. ProjectMergeRule currently refuses to reduce identity projects if the fields have different names.

For instance suppose you have a table Dept (deptno, name) and the algebra

  2: Project($1 as X, $0 as Y)
    1: Project($1, $0)
      0: Scan(Dept)

Observe that if you combine projects #1 and #2 you end up with

  3: Project($0 as X, $1 as Y)
    0: Scan(Dept)

Although the new project (#3) is an identity, it renames the fields. ProjectMergeRule will return the new project (#3), but it could return Scan(Dept) (#0). I think the latter is better, so that will be the new behavior.