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https://issues.apache.org/jira/browse/CALCITE-3865?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17062975#comment-17062975
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Xiening Dai commented on CALCITE-3865:
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I can see the issue with column mapping here. But why do you believe
canConvert() should always return true? The statement in the current logic
still holds -
"Returns true only if we can convert. In this case, we can only convert if the
fromTrait (the input) has fields that the toTrait wants to sort."
Or you think we would never have cases where the input doesn't contain fields
to sort?
> RelCollationTraitDef.canConvert should always return true
> ---------------------------------------------------------
>
> Key: CALCITE-3865
> URL: https://issues.apache.org/jira/browse/CALCITE-3865
> Project: Calcite
> Issue Type: Improvement
> Components: core
> Reporter: Haisheng Yuan
> Priority: Major
> Attachments: image-2020-03-19-16-44-25-621.png,
> image-2020-03-19-16-47-21-805.png
>
>
> CALCITE-1148 introduced the following change to RelCollationTraitDef to fix
> RelTrait conversion bug, but it is just hiding the underlying issue and
> adding redundant and unnecessary check to planner.
> {code:java}
> @Override public boolean canConvert(RelOptPlanner planner,
> RelCollation fromTrait, RelCollation toTrait, RelNode fromRel) {
> // Returns true only if we can convert. In this case, we can only convert
> // if the fromTrait (the input) has fields that the toTrait wants to sort.
> for (RelFieldCollation field : toTrait.getFieldCollations()) {
> int index = field.getFieldIndex();
> if (index >= fromRel.getRowType().getFieldCount()) {
> return false;
> }
> }
> return true;
> }
> {code}
> The root cause is that logical operators, especially LogicalSort can have
> traits, which is a bad design decision, and {{AggregateReduceFunctionsRule}}
> and {{RelBuilder}} fails to adjust the column mapping in RelTraitSet. The
> newly created {{LogicalProject}} has collation on column 5 (it just copy its
> input's RelTraitSet blindly), but it only has 2 columns.
> !image-2020-03-19-16-44-25-621.png!
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