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https://issues.apache.org/jira/browse/CALCITE-5332?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17617979#comment-17617979
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Stamatis Zampetakis commented on CALCITE-5332:
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[~julianhyde] I am bit surprised to hear that extending a rule via the DEFAULT
config is an anti-pattern. Actually at the moment the vast majority of built-in
rules come with at least one DEFAULT config instance. The pruning rules are a
small exception to the general pattern. Moreover, the Javadoc of the
{{RelRule}} interface explicitly asks to [create a DEFAULT configuration
instance|https://github.com/apache/calcite/blob/2c30a56158cdd351d35725006bc1f76bb6aac75b/core/src/main/java/org/apache/calcite/plan/RelRule.java#L86].
> 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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