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https://issues.apache.org/jira/browse/CALCITE-5401?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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ASF GitHub Bot updated CALCITE-5401:
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Labels: pull-request-available (was: )
> Rule fired by HepPlanner can return Volcano's RelSubset
> -------------------------------------------------------
>
> Key: CALCITE-5401
> URL: https://issues.apache.org/jira/browse/CALCITE-5401
> Project: Calcite
> Issue Type: Bug
> Components: core
> Reporter: Ruben Q L
> Priority: Major
> Labels: pull-request-available
>
> TLDR; {{CoreRules.AGGREGATE_REMOVE}} is fired by a {{HepPlanner}} but while
> removing the Aggregate, instead of returning the Aggregate's input, it
> returns a VolcanoPlanner's RelSubset with the input, which leads to
> unforeseeable consequences.
>
> Details: This seems a strange issue that happens because several factors
> occur.
> I first reproduced it on my application with the following query (on TPCH):
> {code:sql}
> SELECT c.c_custkey
> FROM customer c
> WHERE c.c_name IN ('271', '272', '273', '274', '275', '276', '342', '343',
> '344', '345','346', '347', '348', '349', '350', '351', '352', '353', '354',
> '355', '356', '357', '358', '359', '360')
> AND EXISTS (SELECT 1 FROM orders o WHERE o.o_custkey = c.c_custkey)
> {code}
> But the issue can be reproduced also in Calcite by adding this test into
> {{{}HepPlannerTest{}}}:
> {code:java}
> @Test void testAggregateRemove() {
> final RelBuilder builder = RelBuilderTest.createBuilder(c ->
> c.withAggregateUnique(true));
> final RelNode root =
> builder
> .values(new String[]{"i"}, 1, 2, 3) // important to have values
> sorted
> .distinct()
> .build();
> final HepProgram program = new HepProgramBuilder()
> .addRuleInstance(CoreRules.AGGREGATE_REMOVE)
> .build();
> final HepPlanner planner = new HepPlanner(program);
> planner.setRoot(root);
> final RelNode result = planner.findBestExp();
> assertThat(result, is(instanceOf(LogicalValues.class))); // fails because
> result is a RelSubset
> }
> {code}
> The important elements are: firstly our {{RelOptCluster}} has a
> {{VolcanoPlanner}} as planner (so any {{relNode.getCluster().getPlanner()}}
> call that we execute will return a {{VolcanoPlanner}} instance). Nevertheless
> we also apply some rules via a {{{}HepPlanner{}}}. I think this is a quite
> common strategy in Calcite clients to obtain a better performance: first
> apply a subset of rules that are always beneficial via a {{{}HepPlanner{}}},
> and then apply the "main" set of rules via the cost-based
> {{{}VolcanoPlanner{}}}.
> Secondly, we have {{{}AggregateRemoveRule{}}}, which we use in the
> {{HepPlanner}} phase.
> This rule contains the following code:
> {code:java}
> @Override public void onMatch(RelOptRuleCall call) {
> final Aggregate aggregate = call.rel(0);
> final RelNode input = aggregate.getInput();
> ...
> final RelNode newInput = convert(input,
> aggregate.getTraitSet().simplify()); // <-- *** [1]
> relBuilder.push(newInput);
> ...
> call.getPlanner().prune(aggregate); // <-- *** [2]
> call.transformTo(relBuilder.build());
> }
> {code}
> Notice the line [2] which uses {{call.getPlanner()}} to call the {{prune}}
> method. By using {{call.getPlanner()}} we get the correct planner of the rule
> that is being fired, in this case a {{{}HepPlanner{}}}, so we end up calling
> {{{}HepPlanner#prune{}}}, which is fine.
> However, the line [1] calls the {{RelOptRule#convert}} static method:
> {code:java}
> public static RelNode convert(RelNode rel, RelTraitSet toTraits) {
> RelOptPlanner planner = rel.getCluster().getPlanner(); // <-- *** [3]
> RelTraitSet outTraits = rel.getTraitSet();
> for (int i = 0; i < toTraits.size(); i++) {
> RelTrait toTrait = toTraits.getTrait(i);
> if (toTrait != null)
> outTraits = outTraits.replace(i, toTrait);
> }
> if (rel.getTraitSet().matches(outTraits))
> return rel;
> return planner.changeTraits(rel, outTraits); // <-- *** [4]
> }
> {code}
> Notice how in this case, the planner is obtained from the relNode's cluster
> [3], in our case that would be the {{{}VolcanoPlanner{}}}, which is
> potentially problematic. Further down, if the relNode matches the
> {{{}outTraits{}}}, no action is done and the same relNode is returned, no
> problem here. But, if it does not match them, then
> {{RelOptPlanner#changeTraits}} will be called, i.e.
> {{VolcanoPlanner#changeTraits}} [4], and this is where the problem will
> originate: in our scenario {{VolcanoPlanner#changeTraits}} will return a
> Volcano's {{{}RelSubset{}}}, which is completely unhandable by the
> {{HepPlanner}} that triggered the rule, and that leads to the incorrect plan
> returned by the {{{}HepPlanner{}}}.
> In this case, what happens with our original query ({{{}LogicalValues{}}}
> with sorted values), we get to {{RelOptRule#convert}} with the RelNode being
> a {{LogicalValues}} with {{Convention.NONE}} + {{{}Collation[0]{}}}, and the
> {{toTraits}} are the ones from the {{LogicalAggregate}} that we are removing:
> {{Convention.NONE}} + {{Collation[]}} . Since the traits from the
> {{LogicalValues}} do not match the LogicalAgggregate traits ({{{}Collation[0]
> != Collation[]{}}}), the {{RelOptPlanner#changeTraits}} is called and the
> problem occurs. I am not sure why here {{RelTraitSet#matches}} is used (which
> computes an exact match, hence returning false), rather than
> {{{}RelTraitSet#satisfies{}}}, which would have returned true, because a
> sorted {{LogicalValues}} ({{{}Collation[0]{}}}) satisfies the unsorted
> {{{}Collation[]{}}}, but I assume there is a reason for that.
> As a workaround, if the {{LogicalValues}} elements are NOT in order, then the
> problem is avoided: we deal with a {{LogicalValues}} with {{Collation[]}} ,
> so inside {{RelOptRule#convert}} the {{LogicalValues}} traits
> ({{{}Convention.NONE + Collation[]{}}}) match the {{LogicalAggregates}} ones
> ({{{}Convention.NONE + Collation[]{}}}), so the method returns without
> calling {{{}RelOptPlanner#changeTraits{}}}, so the problem does not happen.
> This can be confirmed by modifying the proposed test:
> {code:java}
> @Test void testAggregateRemoveOk() {
> final RelBuilder builder = RelBuilderTest.createBuilder(c ->
> c.withAggregateUnique(true));
> final RelNode root =
> builder
> .values(new String[]{"i"}, 1, 42, 3) // not sorted
> .distinct()
> .build();
> final HepProgram program = new HepProgramBuilder()
> .addRuleInstance(CoreRules.AGGREGATE_REMOVE)
> .build();
> final HepPlanner planner = new HepPlanner(program);
> planner.setRoot(root);
> final RelNode result = planner.findBestExp();
> assertThat(result, is(instanceOf(LogicalValues.class))); // ok
> }
> {code}
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