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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