liyafan82 opened a new pull request #1839: [CALCITE-3836] The hash codes of RelNodes are unreliable URL: https://github.com/apache/calcite/pull/1839 For all sub-classes of AbstractRelNode, the hashCode methods depend on AbstractRelNode#hashCode, because it is declared as final. AbstractRelNode#hashCode depends on Object#hashCode, which is called identify hash code. The details of identity hash code depends on the specific JVM implementation. For many JVMs, the implementation is based on the object address in the memory. The problem is that, the address of an object may change in a JVM, due to GC, memory contraction, etc. So the hash code of an object may change, even if the content of the object is not changed (This can be confirmed from the JavaDoc of Object#hashCode). This problem may cause severe issues that are hard to diagnose and debug, like an object is in the hash table, but cannot be retrieved; duplicate objects in the hash map, etc. To solve the problem, we compute a hash code solely from the node id. This is consistent with the previous semantics, and solves the above problem.
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