[jira] [Created] (CALCITE-3774) Add option to RelBuilder to prevent it from merging projects

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

Julian Hyde created CALCITE-3774:
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             Summary: 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


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)}} followed by
* Input 2: {{Project(w*x AS p, x*y AS q, y*z AS r)}} 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). The inputs have complexity 28 (4 expressions, each of complexity 7) and 9 (3 expressions, each with complexity 3). The 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)
* 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.



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