waynexia opened a new pull request #792:
URL: https://github.com/apache/arrow-datafusion/pull/792
# Which issue does this PR close?
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Resolve #566.
# Rationale for this change
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This pull request trying to implement the common subexpression eliminate
optimization. The current implementation only supports detecting & eliminating
common subexpressions under one logical plan, not cross plans.
This optimizes is consists of two parts: detecting and replacing. It first
scans all expressions we are going to rewrite (all expressions under one plan
here), generates identifiers for each `Expr` node for later comparison. Then
for the same group of expressions, we extract common expressions, put them into
a `Projection` plan, and replace the original expression with a column `Expr`.
This implementation doesn't cover the situation of eliminating the whole
plan's common subexpression. This requires considering not only the expression
itself, but also sharing data with different plans or some plan will modify the
data (e.g. filter) or other aspects I haven't realize. In the beginning I plan
to make it cross plans but it seems better to me to do them in another PR.
# What changes are included in this PR?
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Add optimize rule `CommonSubexprEliminate`.
# Are there any user-facing changes?
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I think there is, as the optimization rule will rewrite input SQL.
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# Status
This is still a work in progress. I have only covered part of plans and am
trying to make it work with the entire execution procedure.
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