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https://issues.apache.org/jira/browse/CALCITE-3878?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17068428#comment-17068428
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neoremind edited comment on CALCITE-3878 at 3/27/20, 8:54 AM:
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[~donnyzone] For code style, I think that is a good advice but not mandatory.
But for critical path or hot code, people should aware of that potential
overhead.
Assume we do know the exact size of a list or map (plus loading factor), if the
collection does not return or goes into deeper method, then we can tell it is
100% safe without resizing. If the collection escapes and being manipulated
again, simply let it resize because it works the same as we don't specify
initial capacity currently. That's how STL or Java collection should work.
I sorry I am little sensitive to memory usage and performance, compared with IO
overhead, this is too little to talk. Fundamental framework should balance good
code and performance well, I think the change of making an initial capacity
does not disturbing too much. Personal opinion :D
was (Author: neoremind):
[~donnyzone] For code style, I think that is a good advice but not mandatory.
But for critical path or hot code, people should aware of that potential
overhead.
Assume we do know the exact size of a list or map (plus loading factor), if the
collection does not return or goes into deeper method, then we can tell it is
100% safe without resizing. If the collection escapes and being manipulated
again, simply let it resize because it works the same as we don't specify
initial capacity currently.
I sorry I am little sensitive to memory usage and performance, compared with IO
overhead, this is too little to talk. Fundamental framework should balance good
code and performance well, I think the change of making an initial capacity
does not disturbing too much. Personal opinion :D
> Make ArrayList creation with initial capacity when size is fixed
> ----------------------------------------------------------------
>
> Key: CALCITE-3878
> URL: https://issues.apache.org/jira/browse/CALCITE-3878
> Project: Calcite
> Issue Type: Improvement
> Components: core
> Affects Versions: 1.22.0
> Reporter: neoremind
> Priority: Minor
> Labels: pull-request-available
> Time Spent: 40m
> Remaining Estimate: 0h
>
> I find many places in Calcite where _new ArrayList<>()_ is used, if the list
> is expected to be immutable or not resizing, it is always a good manner to
> create with initial capacity, better for memory usage and performance.
> I search all occurrences, focus on the core module, to make it safe, I only
> update local variables with fixed size and not working in recursive method.
> If the local variable reference goes out of scope, if resizing is needed,
> things will work normally as well, so no side effect, but for the "escaping"
> case, I am very conservative and do not change them.
>
>
>
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