[jira] [Created] (CALCITE-808) Optimize ProjectMergeRule

["Julian Hyde (JIRA)" <ji...@apache.org>]

Julian Hyde created CALCITE-808:
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             Summary: Optimize ProjectMergeRule
                 Key: CALCITE-808
                 URL: https://issues.apache.org/jira/browse/CALCITE-808
             Project: Calcite
          Issue Type: Bug
            Reporter: Julian Hyde
            Assignee: Julian Hyde


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

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

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

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

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.



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