Answer Posted / vishal

UNION removes duplicates, whereas UNION ALL does not.

In order to remove duplicates the result set must be
sorted, and this may have an impact on the performance of
the UNION, depending on the volume of data being sorted,
and the settings of various RDBMS parameters ( For Oracle
PGA_AGGREGATE_TARGET with WORKAREA_SIZE_POLICY=AUTO or
SORT_AREA_SIZE and SOR_AREA_RETAINED_SIZE if
WORKAREA_SIZE_POLICY=MANUAL ).

Basically, the sort is faster if it can be carried out in
memory, but the same caveat about the volume of data
applies.

Of course, if you need data returned without duplicates
then you must use UNION, depending on the source of your
data.

I would have commented on the first post to qualify the "is
much less performant" comment, but have insufficient
reputation (points) to do so.

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