PaulJackson123 commented on code in PR #3495:
URL: https://github.com/apache/calcite/pull/3495#discussion_r1408236861
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core/src/main/java/org/apache/calcite/rel/metadata/BuiltInMetadata.java:
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@@ -83,9 +83,9 @@ public interface UniqueKeys extends Metadata {
* represented as an {@link org.apache.calcite.util.ImmutableBitSet}, where
* each bit position represents a 0-based output column ordinal.
*
- * <p>Note that a key plus other columns is still unique. Therefore a table
- * with all columns unique has a unique key consisting of the empty set.
- * This is the case with all single-row tables.
+ * <p>Note that a unique key plus other columns is still unique.
+ * Conversely, all columns are unique in a table with a unique key
+ * consisting of the empty set, as is the case for all single-row tables.
Review Comment:
No, this is good. I see your point now. In retrospect, your zero-column
argument should have been sufficient. My confusion went like this:
1. If (A) is a unique key, (A,B) is also a unique key
2. A 0-or-1-row table has () as a unique key
3. All columns in a table with () as a key are unique
4. A table with all columns unique has () as a key
I didn't catch the leap between 3 and 4. I've been quietly unsettled about
empty keys and this clarification helps with that.
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