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Radu Berinde (RaduBerinde) commented:
The docs on set operations only mention this much on DISTINCT vs ALL:
By default, each of these comparisons displays only one copy of each value (similar to SELECT DISTINCT). However, each function also lets you add an ALL to the clause to display duplicate values.
The semantics of ALL are more complicated than this. For example, if the same row appears on the left side 5 times and on the right side 2 times, EXCEPT ALL would remove 2 of the 5 occurrences. Similarly INTERSECT ALL would keep 2 occurrences.
The doc should also make it more explicit that DISTINCT is the same with not specifying ALL/DISTINCT (e.g. EXCEPT is the same with EXCEPT DISTINCT).
Jira Issue: DOC-449