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https://issues.apache.org/jira/browse/CALCITE-4559?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17324605#comment-17324605
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Vladimir Ozerov edited comment on CALCITE-4559 at 4/19/21, 8:19 AM:
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The design looks great to me in general. I would like to highlight two
potential improvements:
# Patterns might be extremely complicated. In our practice, we observed
situations like "the disjunctive expression that have attributes from two
inputs on the one side and non-equi condition on the other side". The current
approach is not flexible enough - most classes are hidden from the user, and
the composition of static checks might be not sufficient. In the general case,
a user might need to execute a complex visitor on the expression, to check
whether it matches the rule. I would suggest defining a public pattern
interface that allows for user-defined implementations. The
{{OperandDetailBuilder}} might be a convenient basic implementation for the
user, but not the only option.
# Rex patterns might be useful outside of the {{RexRule}}. For example, a user
might want to implement a {{RelRule}} that matches a {{Filter}} with very
special requirements to the condition. Converting {{RexRuleProgram.Operand}} to
a separate public interface (e.g., {{RexPattern}}) might be very helpful.
Also, heuristic rule programs are very sensitive to the order of rule
application. Currently, the rules are applied bottom-up only. There is a chance
that in the future we may need alternative strategies. But perhaps it could be
put out of the scope for now.
was (Author: vozerov):
The design looks great to me in general. I would like to highlight two
potential improvements:
# Patterns might be extremely complicated. In our practice, we observed
situations like "the disjunctive expression that have attributes from two
inputs on the one side and non-equi condition on the other side". The current
approach is not flexible enough - most classes are hidden from the user, and
the composition of static checks might be not sufficient. In the general case,
a user might need to execute a complex visitor on the expression, to check
whether it matches the rule. I would suggest defining a public pattern
interface that allows for user-defined implementations. The
{{OperandDetailBuilder}} might be a convenient basic implementation for the
user, but not the only option.
# Rex patterns might be useful outside of the {{RexRule}}. For example, a user
might want to implement a {{RelRule}} that matches a {{Filter}} with very
special requirements to the condition. Converting {{RexRuleProgram.Operand}} to
a separate public interface (e.g., {{RexPattern}}), might be very helpful.
Also, heuristic rule programs are very sensitive to the order of rule
application. Currently, the rules are applied bottom-up only. There is a chance
that in the future we may need alternative strategies. But perhaps it could be
put out of the scope for now.
> Create 'interface RexRule', a modular rewrite for row-expressions
> -----------------------------------------------------------------
>
> Key: CALCITE-4559
> URL: https://issues.apache.org/jira/browse/CALCITE-4559
> Project: Calcite
> Issue Type: Bug
> Reporter: Julian Hyde
> Assignee: Julian Hyde
> Priority: Major
>
> We propose to add {{class RexRule}}, a rewrite rule for row-expressions
> ({{class RexNode}}).
> {{class RexRule}} is analogous to how {{class RelRule}} (and the older
> {{class RelOptRule}}) operates on relational expressions ({{interface
> RelNode}}). Also, {{class RexRuleProgram}} is analogous to {{HepProgram}} and
> {{VolcanoPlanner}} (it indexes rules so that we do not have to try every rule
> against every part of the expression). And a rule describes which operands it
> matches using {{RexRule.describe(RexRule.OperandBuilder)}}, similar to
> calling {{RelRule.Config.operandSupplier().apply()}}.
> The advantages of {{RexRule}} are similar to {{RelRule}}: rules can be
> defined in a modular way, can be documented and tested individually, and can
> be enabled individually.
> The rules could be applied in various ways. {{RelBuilder.Config}} could
> contain a {{RexRuleProgram}} that would be applied every time an expression
> is simplified by a {{RelBuilder}}. There could also be a sub-class of
> {{interface RelShuttle}} that applies the rules to every {{RexNode}} in a
> tree (e.g. inside {{Filter}}, {{Project}} and {{Join}}).
> I don't yet know whether, or how, rules might support 3-valued boolean logic
> ({{RexUnknownAs}}). For example, a rule that simplifies "x = x" to "TRUE" is
> valid in an "unknownAsFalse" context (e.g. as top-level of {{Filter}}
> condition), but not in an "unknownAsUnknown" context (e.g. in {{Project}}
> expression).
> This case is related to CALCITE-3470 (making relational and row-expression
> rules more similar, as in CockroachDB), but would deliver an API rather than
> a textual DSL.
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