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https://issues.apache.org/jira/browse/CALCITE-808?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14635905#comment-14635905
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Julian Hyde commented on CALCITE-808:
-------------------------------------
Note that if you want the old behavior, rather than using
ProjectMergeRule.INSTANCE, you should create your own instance with force =
false: {code}new ProjectMergeRule(false,
RelFactories.DEFAULT_PROJECT_FACTORY){code}
> 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
> Fix For: 1.4.0-incubating
>
>
> 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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