zabetak commented on code in PR #4013:
URL: https://github.com/apache/calcite/pull/4013#discussion_r1820905037
##########
core/src/main/java/org/apache/calcite/rel/metadata/RelMdUniqueKeys.java:
##########
@@ -184,19 +180,12 @@ private static Set<ImmutableBitSet>
getProjectUniqueKeys(SingleRel rel, RelMetad
continue;
}
// colMask is mapped to output project, however, the column can be
mapped more than once:
- // select id, id, id, unique2, unique2
- // the resulting unique keys would be {{0},{3}}, {{0},{4}},
{{0},{1},{4}}, ...
-
- Iterable<List<ImmutableBitSet>> product =
- Linq4j.product(
- Util.transform(colMask, in ->
- Util.filter(
- requireNonNull(mapInToOutPos.get(in),
- () -> "no entry for column " + in
- + " in mapInToOutPos: " +
mapInToOutPos).powerSet(),
- bs -> !bs.isEmpty())));
-
- resultBuilder.addAll(Util.transform(product, ImmutableBitSet::union));
+ // select key1, key1, val1, val2, key2 from ...
+ // the resulting unique keys would be {{0},{4}}, {{1},{4}}
Review Comment:
I am afraid that the above suggestion is incorrect. The `UniqueKeys`
metadata does not return record/row values but column ordinals.
To understand these comments it is important to have read the Javadoc of the
`UniqueKeys` interface before.
`{0}` refers to the column at position zero i.e., the first occurrence of
column key1 in the query.
`{1}`refers to the column at position one i.e., the second occurrence of
column key1 in the query.
...
`{4}`refers to the column at position four i.e., column key2 in the query.
If this is not clear from the Javadoc of the `UniqueKeys` interface then we
should improve that part of the documentation and not this internal low-level
comments. The part here assumes that the developer understands what the result
of this method is.
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