SelectExpression.VisitChildren optimizations #17594
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Do not allocate and copy the different components if visitation hasn't changed anything. Affects #17455
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LGTM as long as test pass. Requested review from Andriy just in case. |
AndriySvyryd
reviewed
Sep 3, 2019
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| changed |= projection != item; | ||
| if (newProjections != _projection) |
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Suggested change
| if (newProjections != _projection) | |
| if (changed) |
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Careful, changed tracks if any component of the SelectExpression was changed, including previous unrelated components. This change seems like it would make us falsely go into the condition.
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But it should be fine for these two cases, since changed is not mutated before
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Ah I see... I'd rather leave it like this - who knows if we'll shift the code fragments around and this would become invalid... There's no real gain in replacing a single reference comparison with a bool check...
AndriySvyryd
reviewed
Sep 3, 2019
AndriySvyryd
approved these changes
Sep 3, 2019
ghost
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Profiling the scenario in #17455 showed that SelectExpression.VisitChildren was a major compilation perf hotspot (own time, not including child calls). This PR changes the logic to only allocate new data structures and copy if a Visit actually returned a different result.
This reduced the time for #17455 from 22 seconds to 15 on my machine. Memory allocated notably goes down from 61.86MB to 12.17MB.
Also added a compilation micro-benchmark which does three joins, here are the results. Note that #17455 includes 6 results and so the difference is considerably more significant.
Before
After