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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