Predict output

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Predict output

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Frédéric van der Plancke SSC Enthusiast Points: 198 More actions · Answer 1

I do NOT think that "union (distinct)" always entails an "order by", neither theoretically (as several others have pointed out), neither practically.

While I've done no testing, there may be cases where SQLServer resorts to hash comparisons insead of ordering to eliminate duplicates. You're probably 99% safe assuming it will "order by" a single int field union. In more complex situations, I wouldn't bet (yet).

Is there any written guarantee from MS that any version of MSSQL always returns ordered results out of "union (disjoint)" queries ? Or are you people just wildguessing ?

Adam Machanic SSCoach Points: 15370 More actions · Answer 2

As others have mentioned, a sort is not the only option, and the NULL may or may not come first... No one actually showed this yet, though, so here you go:

SELECT 1 UNION SELECT NULL UNION SELECT '1'

OPTION (HASH UNION)

--
Adam Machanic
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Piotr.Rodak SSCrazy Eights Points: 9376 More actions · Answer 3

Nice question.

The execution plan for slightly modified query

SELECT 2 UNION

SELECT 1 union

SELECT NULL UNION

SELECT '1' union

SELECT '2'

shows that Merge Join operators are used to merge inputs from particular SELECT queries. BOL says that Merge Join operator may introduce sort as it requires inputs to be sorted. Since the Selects return only single values, they are 'sorted' and we don't see explicit sort in execution plan.

Interesting thing happens when I modify the query to following code:

SELECT 2 UNION

select a

from

(

SELECT '2' a UNION ALL

SELECT 1

) q UNION

SELECT NULL UNION

SELECT '1'

The Merge Joins are replaced by Concatenation operator and explicit Distinct Sort is used to remove duplicates. If I add option (merge union) to above query, the sort operation is added on the input of the Merge Join implemening union SELECT 2 UNION

select a

from ....

I wonder if it is possible to get output 1, NULL, 2 or 2, NULL, 1. The Hash Match operator that is used when option (hash union) is used apparently sorts data anyway, just NULLs are added to the end of the output.

Piotr

...and your only reply is slàinte mhath

TomThomson SSC Guru Points: 104773 More actions · Answer 4

Piotr Rodak (11/22/2008)
The Hash Match operator that is used when option (hash union) is used apparently sorts data anyway, just NULLs are added to the end of the output.
Piotr

The hash union sorts the hash values (and sometimes sorts the values within a set sharing a hash value); presumably the hash value currently used for NULL sorts high. There's no guarantee that it will sort high in the next version. There's no guarantee that the optimier won't chose some other duplicate elimination algorithm which hasn't been mentioned yet when it thinks its appropriate for the dataset, which may end up with a different order.

Tom

kapil_kk SSC-Insane Points: 21316 More actions · Answer 5

Good Question 🙂

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