[
https://issues.apache.org/jira/browse/CALCITE-3774?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17031140#comment-17031140
]
Julian Hyde commented on CALCITE-3774:
--------------------------------------
I regard a call with 5 arguments as more complex than a call with 3 arguments.
Therefore I am defining complexity to be total node count. In your scheme, both
would have complexity 1. In my scheme (assuming that the arguments are leaves)
one would have complexity 6 and the other 4.
In your scheme, {{Project(DEPTNO, 1)}} would have complexity 0. That will cause
problems when you try to compute ratio of new to old complexity.
I was thinking of allowing complexity to increase by up to {{mergeBloat}} if it
is positive. Allow a small increase in complexity because you've got rid of a
{{Project}}.
> Add option to RelBuilder to prevent it from merging projects
> ------------------------------------------------------------
>
> Key: CALCITE-3774
> URL: https://issues.apache.org/jira/browse/CALCITE-3774
> Project: Calcite
> Issue Type: Bug
> Reporter: Julian Hyde
> Priority: Major
>
> Add option to {{RelBuilder}} to prevent it from merging projects. Currently
> if you call {{RelBuilder.project}} and the input is a {{Project}} it will
> merge the expressions. This is usually a good idea, but sometimes it creates
> very complex expressions.
> There is an existing method {{RelBuilder.shouldMergeProject()}} but by
> default it returns true, and in order to change it you have to sub-class
> {{RelBuilder}}, which is not easy to do.
> I propose to add a property {{RelBuilder.Config.mergeBloat}}, default 0,
> which would prevent creating a project that has more complexity than the two
> projects that went into it.
> Example 1:
> * Input 1: {{Project(a+b+c+d AS w, b+c+d+e AS x, c+d+e+f AS y, d+e+f+g AS
> z)}} (complexity 28), followed by
> * Input 2: {{Project(w*x AS p, x*y AS q, y*z AS r)}} (complexity 9) creates
> * Output: {{Project((a+b+c+d) * (b+c+d+e) AS p, (b+c+d+e) * (c+d+e+f) AS q,
> (c+d+e+f) * (d+e+f+g) AS r)}}.
> The expression "a+b+c+d" has complexity 7 (4 fields and 3 calls). Input 1 has
> complexity 28 (4 expressions, each of complexity 7); input 2 has complexity 9
> (3 expressions, each with complexity 3). Output has complexity 45 (3
> expressions, each with complexity 15 (8 fields and 7 calls)). 45 is greater
> than 37 (28 + 9), so this merge would not be allowed.
> Example 2:
> * Input 1: {{Project(a+b+c+d AS w, b+c+d+e AS x, c+d+e+f AS y, d+e+f+g AS
> z)}} (complexity 28), followed by
> * Input 2: {{Project(w*x AS p, x*y AS q)}} (complexity 6) creates
> * Output: {{Project((a+b+c+d) * (b+c+d+e) AS p, (b+c+d+e) * (c+d+e+f) AS
> q)}} (complexity 30).
> Output complexity 30 is less than input complexity 34 (28 + 6), and therefore
> the merge is allowed.
--
This message was sent by Atlassian Jira
(v8.3.4#803005)
