askalt opened a new pull request, #12950:
URL: https://github.com/apache/datafusion/pull/12950
## Which issue does this PR close?
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Closes #12738.
## Rationale for this change
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I investigated the performance degradation in creating physical plans for
queries with a large number of columns compared to version 40 and discovered
the following:
1. The main time loss occurs during the cloning of plan nodes during
optimizations. We can compare two flame graphs (for version 40 and version 42,
attached) and see that in version 42, `enforce_distribution` spends additional
time destroying the vector of AggregateFunExpr. It turns out that in patch
f5c47fa274d53c1d524a1fb788d9a063bf5240ef, the Arc for storing aggregate
expressions was removed. As a result, during calls to `with_new_children`,
fairly heavy structures are being cloned and destroyed.
2. Some time was also spent on a new optimization: limit pushdown. This is
optional and left up to the user to decide whether to use it, so there are no
issues here.
This patch restores the Arc wrappers for storing aggregates in the physical
plan nodes, and also adds a benchmark aimed at preventing the degradation from
recurring.
As can be seen from the flame graphs, a significant amount of time is spent
on creating ProjectionMapping. This mapping is only needed when there are eq
properties, which are irrelevant for some plans, so it gets built
unnecessarily, wasting time. I will raise a separate issue for this.
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I uploaded flamegraphs here https://file.io/x3InMBwSYFs7 (created with
scripts from kernel sources perf tool), because github does not support html
files.
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