julianhyde commented on issue #1229: [CALCITE-3085] Remove RelShuttleImpl#stack protected field URL: https://github.com/apache/calcite/pull/1229#issuecomment-496681425 Has this issue shown up empirically? The stack is implemented as an `ArrayDeque`, so push and pop should be very few machine instructions. `RelShuttleImpl` is not the only possible implementation of `RelShuttle`. It is just a good general-purpose implementation. If it doesn't meet your needs, you can create your own. No sub-classes in Calcite use the `stack` field, but it's there, and it is potentially useful, if I am looking for say a `Filter` within a `Project` within a `Filter`. Consider `AbstractList` class in the JDK. It contains a `modCount` field is useful for most implementations (prevents concurrent modifications) but is unnecessary overhead for sub-classes that are immutable. To avoid that overhead, I created `AbstractImmutableList`, but I wouldn't suggest removing `modCount` from `AbstractList`.
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