[jira] [Commented] (DRILL-3912) Common subexpression elimination in code generation

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

    [ https://issues.apache.org/jira/browse/DRILL-3912?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14947733#comment-14947733 ] 

Julian Hyde commented on DRILL-3912:
------------------------------------

The search space for common relational expressions is so much larger (and so different) that it needs a different approach. In Calcite we are planning to introduce a Spool operator to create temporary tables and then re-use them throughout the query as if they were materialized views.  Furthermore the temporary tables might be virtual (i.e. never fully materialized, but have two or subscribers to the stream of records).

CALCITE-481 is the placeholder for that work.

> Common subexpression elimination in code generation
> ---------------------------------------------------
>
>                 Key: DRILL-3912
>                 URL: https://issues.apache.org/jira/browse/DRILL-3912
>             Project: Apache Drill
>          Issue Type: Bug
>            Reporter: Steven Phillips
>            Assignee: Steven Phillips
>
> Drill currently will evaluate the full expression tree, even if there are redundant subtrees. Many of these redundant evaluations can be eliminated by reusing the results from previously evaluated expression trees.
> For example,
> {code}
> select a + 1, (a + 1)* (a - 1) from t
> {code}
> Will compute the entire (a + 1) expression twice. With CSE, it will only be evaluated once.
> The benefit will be reducing the work done when evaluating expressions, as well as reducing the amount of code that is generated, which could also lead to better JIT optimization.



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