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https://issues.apache.org/jira/browse/CALCITE-5332?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17617990#comment-17617990
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Julian Hyde commented on CALCITE-5332:
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I had forgotten about that recommendation. I haven't been in that code for a
while.
IIRC, the recommendation to create a {{DEFAULT}} was more about about managing
dependencies (i.e. creating a sane order to load classes) and avoiding
duplicated code (if there are several similar instances of a rule). I think
that it would be OK if a {{DEFAULT}} were package-private, for example.
I'm not laying down absolute rules here, but the general pattern we want to
encourage is for users to use an instance of a rule to create a new instance of
a rule.
It was a mistake to make the rule instances have type {{RelOptRule}} rather
than {{RelRule}} or some more specific sub-class.
> Configuring PruneEmptyRules is cumbersome
> -----------------------------------------
>
> Key: CALCITE-5332
> URL: https://issues.apache.org/jira/browse/CALCITE-5332
> Project: Calcite
> Issue Type: Bug
> Components: core
> Affects Versions: 1.32.0
> Reporter: Stamatis Zampetakis
> Assignee: Stamatis Zampetakis
> Priority: Major
> Labels: pull-request-available
> Fix For: 1.33.0
>
> Time Spent: 20m
> Remaining Estimate: 0h
>
> After CALCITE-4787/CALCITE-4839 (release 1.28.0) it is pretty complicated to
> create new PruneEmptyRule instances with slightly different configurations.
> The most common use-case is to modify a rule, for instance
> [PruneEmptyProject|https://github.com/apache/calcite/blob/2c30a56158cdd351d35725006bc1f76bb6aac75b/core/src/main/java/org/apache/calcite/rel/rules/PruneEmptyRules.java#L178],
> and change the operands to enlarge or restrict the matching scope; e.g.,
> instead of matching the general {{Project.class}} match only
> {{HiveProject.class}}.
> At the moment this task is not straightforward for various reasons.
> We cannot create the new rule by obtaining the configuration from the
> existing instance (e.g., PROJECT_INSTANCE) cause the latter is declared as
> RelOptRule. Unless we cast the instance to {{RelRule}} we cannot get access
> to its configuration and change it.
> {{ImmutableRemoveEmptySingleRuleConfig}} is package private as every other
> immutable class so again we cannot start from there.
> The constructors of {{RemoveEmptySingleRule}} are either deprecated or
> package private so cannot/should not be used.
> [RemoveEmptySingleRuleConfig|https://github.com/apache/calcite/blob/2c30a56158cdd351d35725006bc1f76bb6aac75b/core/src/main/java/org/apache/calcite/rel/rules/PruneEmptyRules.java#L351]
> does not provide a {{DEFAULT}} configuration instance.
> The only way "recommended" way to create a new rule instance at the moment is
> to implement the respective config interface. This is unnecessary and
> requires quite a bit of work if the consumer is not using the immutables
> library annotation processor.
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