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https://issues.apache.org/jira/browse/CALCITE-2717?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16704927#comment-16704927
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Julian Hyde commented on CALCITE-2717:
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I'm surprised. I thought soft references won't prevent anything from being 
garbage collected; if it's not being garbage collected, it's because there is 
some other reference to it outside of the cache. Look for documentation on how 
to use a guava cache to intern values.

> Cached traits in RelTraitDef's canonicalMap can't be garbage-collected 
> -----------------------------------------------------------------------
>
>                 Key: CALCITE-2717
>                 URL: https://issues.apache.org/jira/browse/CALCITE-2717
>             Project: Calcite
>          Issue Type: Bug
>          Components: core
>            Reporter: Haisheng Yuan
>            Assignee: Julian Hyde
>            Priority: Major
>
> Softly-referenced objects will be garbage-collected in a globally 
> least-recently-used manner, in response to memory demand. Even though 
> canonicalMap's value is soft referenced, key is strong referenced, key and 
> value are referencing the same object. So traits in the cache will never be 
> garbage-collected, which may cause OOM if we have tons of different traits.



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