Does SQL Server CASE expression not do short circuit? - sql-server

Question

Appropriate if somebody could help us to optimize below query
As per execution plane it seems the else part sub query is always executing, irrespective of conditions.
Won't CASE be short circuited? Why is it executing, even though it is not necessary?
IF OBJECT_ID('#Calculation') IS NOT NULL
DROP TABLE #Calculation;
SELECT
Result.IdDeckungsbeitrag,
Result.wert AS Wert,
REPLACE(#Formula, '<#PackagingCosts> ', Result.wert) AS Kalkulation
INTO
#Calculation
FROM
(SELECT
deck.IdDeckungsbeitrag,
CASE
WHEN lp.ID_VERPACKUNG_2 IS NULL
AND lp.ID_VERPACKUNG_3 IS NULL
THEN vg.VERPACKUNGSKOSTEN_PRO_EINHEIT * deck.Menge
ELSE
(
SELECT SUM(temp.me) * vg.VERPACKUNGSKOSTEN_PRO_EINHEIT
FROM
(
SELECT SUM(gv.MENGE) AS me
FROM dbo.KUNDENRECHNUNG_POSITION krp
LEFT JOIN dbo.LIEFERSCHEIN_POSITION lp
ON lp.ID_LIEFERSCHEIN_POSITION = krp.ID_LIEFERSCHEIN_POSITION
LEFT JOIN dbo.GEBINDE_VERLADEN gv
ON gv.ID_LIEFERSCHEIN_POSITION = lp.ID_LIEFERSCHEIN_POSITION
LEFT JOIN dbo.MATERIAL_BESTAND mb
ON mb.ID_MATERIAL_BESTAND = gv.ID_MATERIAL_BESTAND
LEFT JOIN dbo.MATERIAL_GEBINDE mg
ON mg.ID_MATERIAL_BESTAND = mg.ID_MATERIAL_BESTAND
WHERE mg.CHARGE_NUMMER = deck.Charge
GROUP BY mg.ID_VERPACKUNG
) temp
)
END AS wert
FROM #DeckungsbeitragCalculationPositions_TVP deck
LEFT JOIN dbo.KUNDENRECHNUNG_POSITION krp
ON krp.ID_KUNDENRECHNUNG_POSITION = deck.IdDeckungsbeitrag
LEFT JOIN dbo.LIEFERSCHEIN_POSITION lp
ON lp.ID_LIEFERSCHEIN_POSITION = krp.ID_LIEFERSCHEIN_POSITION
LEFT JOIN dbo.VERPACKUNG vg
ON vg.ID_VERPACKUNG = lp.ID_VERPACKUNG_1
WHERE deck.IdMandant = #Id_Mandant
) Result;

Answer 1

Don't think of it as short-circuit or not. That's a procedural code mindset rather than a set-based mindset.
In SQL, the actual "execution" happens in the realm of table or index scans and seeks, hash matches, sorts, and the like. Pull data from different tables into a working set that will eventually produce the desired relational result.
Not seeing any real or projected execution plan, in this case (no pun intended), I suspect the query optimizer decided it was most efficient to first produce this result set in memory:
SELECT mg.ID_VERPACKUNG, mg.CHARGE_NUMMER, SUM(gv.MENGE) AS me
FROM dbo.KUNDENRECHNUNG_POSITION krp
LEFT JOIN dbo.LIEFERSCHEIN_POSITION lp
ON lp.ID_LIEFERSCHEIN_POSITION = krp.ID_LIEFERSCHEIN_POSITION
LEFT JOIN dbo.GEBINDE_VERLADEN gv
ON gv.ID_LIEFERSCHEIN_POSITION = lp.ID_LIEFERSCHEIN_POSITION
LEFT JOIN dbo.MATERIAL_BESTAND mb
ON mb.ID_MATERIAL_BESTAND = gv.ID_MATERIAL_BESTAND
LEFT JOIN dbo.MATERIAL_GEBINDE mg
ON mg.ID_MATERIAL_BESTAND = mg.ID_MATERIAL_BESTAND
GROUP BY mg.ID_VERPACKUNG, mg.CHARGE_NUMMER
This is the subquery from the ELSE clause, minus the WHERE clause conditions and with additional info added to the SELECT to make the match more effective. If the query optimizer can't be confident of meeting the WHEN clause a high percentage of the time, it might believe producing this larger ELSE set once to match against as needed is more efficient. Put another way, if it thinks it will have to run that subquery a lot anyway, it may try to pre-load it for all possible data.
We don't know enough about your database to suggest real solutions, but indexing around the ID_VERPACKUNG_2, ID_VERPACKUNG_3, and CHARGE_NUMMER fields might help. You might also be able to use a CTE, temp table, or table variable to help Sql Server to a better job of caching this data just once.

Answer 2

If the conditions of where are not met then the else will become active. If this is always the case then you would expect the following to return no rows:
select *
FROM #DeckungsbeitragCalculationPositions_TVP deck
LEFT JOIN dbo.KUNDENRECHNUNG_POSITION krp ON krp.ID_KUNDENRECHNUNG_POSITION = deck.IdDeckungsbeitrag
LEFT JOIN dbo.LIEFERSCHEIN_POSITION lp ON lp.ID_LIEFERSCHEIN_POSITION = krp.ID_LIEFERSCHEIN_POSITION
WHERE lp.ID_VERPACKUNG_2 IS NULL
AND lp.ID_VERPACKUNG_3 IS NULL