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https://issues.apache.org/jira/browse/CALCITE-3786?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17141268#comment-17141268
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Danny Chen commented on CALCITE-3786:
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> Could you please enlighten me on how to properly customize it?
Just implement the #explainTerms correctly with the items you want to involve
in.
> Or My operator has some additional field, that participate in the operator
> identity, aka digest comparison, but I don't want to output it in the explain
> terms, is there anyway I can achieve this?
Use different SqlExplainLevel to distinguish just like before.
> Object#hashCode and #equals final so that we have a bug-free world.
I think things are much different when there are 3 methods there, especially
when we are talking about "semantic equivalent" not strict #equals. And i'm
confused why we reference the Flink code, to prove what ?
> 4 bytes to the Cons data structure to serve his purpose
Just like Julian said, Cons is a basic data structure, it can be used in many
codes, but digest is a planner internal thing, it is better if we can avoid
theses duplicate object references, but it is not necessary from my side if we
can make the code more concise.
> Add Digest interface to enable efficient hashCode(equals) for RexNode and
> 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
> Assignee: Danny Chen
> Priority: Major
> Fix For: 1.24.0
>
> Time Spent: 8h 40m
> Remaining Estimate: 0h
>
> 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".
> A 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
> String toString(); // e.g. for debugging purposes
> int compareTo(Digest); // e.g. for debugging purposes.
> }
> {code}
> Note how fields in Kotlin are aligned much better, and it makes it easier to
> read:
> {code:java}
> class Digest { // immutable
> val hashCode: Int // speedup hashCode and equals
> val contents: Array<Any> // The values are either other Digest objects or
> Strings
> fun toString(): String // e.g. for debugging purposes
> fun compareTo(other: Digest): Int // e.g. for debugging purposes.
> }
> {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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