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https://issues.apache.org/jira/browse/DRILL-3912?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14947721#comment-14947721
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Ted Dunning commented on DRILL-3912:
------------------------------------
It sounds like this only deals with common sub-expressions in expressions.
A far more significant optimization would be to deal with common
sub-expressions at a larger scale. A classic case is multiple re-use of a
single expression in a common table expression. For instance,
{code}
with x as (select dir0, id from dfs.tdunning.zoom where id < 12),
y as (select id, count(*) cnt from x group by id),
z as (select count(distinct id) id_count from x)
select dir0, x.id, y.cnt from x , y, z where x.id = y.id and y.cnt /
z.id_count > 3
{code}
Without good sub-expression elimination, table zoom will be scanned three
times. Last I heard, DRILL doesn't optimize this away.
> Common subexpression elimination
> --------------------------------
>
> 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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