Liya Fan created CALCITE-3836:
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Summary: The hash codes of RelNodes are unreliable
Key: CALCITE-3836
URL: https://issues.apache.org/jira/browse/CALCITE-3836
Project: Calcite
Issue Type: Bug
Components: core
Reporter: Liya Fan
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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