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https://issues.apache.org/jira/browse/CALCITE-3786?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17035632#comment-17035632
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Julian Hyde commented on CALCITE-3786:
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This idea has a lot of merit.
There is one potential performance downside: spreading pieces of digest all
over the heap, so that walking a digest (for deep equals, for example) would
generate a lot of cache misses. We'd need to be careful not to make the pieces
of the digest too small.
{{Digest}} would need a {{toString()}} method which would only ever be invoked
in a debugger.
It might be helpful if it implemented {{Comparable<Digest>}}.
> Add Digest (HashStrategy?) interface to enable efficient hashCode/equals for
> RexNode, RelNode
> ---------------------------------------------------------------------------------------------
>
> Key: CALCITE-3786
> URL: https://issues.apache.org/jira/browse/CALCITE-3786
> Project: Calcite
> Issue Type: New Feature
> Components: core
> Affects Versions: 1.21.0
> Reporter: Vladimir Sitnikov
> Priority: Major
>
> Current digests for RexNode, RelNode, RelType, and similar cases use String
> concatenation.
> It is easy to implement, however, it has drawbacks:
> 1) String objects cannot be reused. For instance, RexCall has operands,
> however, the digest is duplicated. It causes extra memory use, and extra CPU
> for string copying
> 2) There's no way to have multiple #toString() methods. RelType might need
> multiple digests: "including field names", "excluding field names".
> Suggested resolution might be behind the lines of
> {code:java}
> class Digest { // immutable
> final int hashCode; // speedup hashCode and equals
> final Object[] contents; // The values are either other Digest objects or
> Strings
> }
> {code}
> Then the digest for RexCall could be the bits relevant to RexCall itself +
> digests of the operands (which can be reused as is)
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