Query Optimization using 'except' - sql

Question

I've been lately working on some performance optimization and have been a bit stuck with the below query. Breaking it down, the individual steps don't seem to take very long, but when I run the query as a whole, it takes about 30minutes to complete.
The TABLE has around 100k rows, and the VIEW has around 400k rows, so they're not terribly large. I wasn't sure if I'm just not understanding the EXCEPT logic accurately, and if that's the likely culprit? Would there be an alternative to EXCEPT perhaps?
EDIT - The view itself has about 4 joins and a UNION, so it does have some logic to it.
CREATE TABLE [SCHEMA].[TABLE](
ColumnA [int] IDENTITY(1,1) NOT NULL,
ColumnB [tinyint] NOT NULL,
ColumnC [tinyint] NOT NULL,
ColumnD [int] NULL,
ColumnE [nvarchar](50) NOT NULL,
ColumnF [int] NOT NULL,
ColumnG [nvarchar](250) NULL,
ColumnH [nvarchar](250) NULL,
ColumnI [nvarchar](250) NULL,
ColumnJ [nvarchar](50) NULL,
columnK [nvarchar](400) NULL,
ColumnL [nvarchar](2) NULL,
ColumnM [nvarchar](250) NULL,
ColumnN [nvarchar](3) NULL,
----
DELETE FROM [DB].[SCHEMA].[TABLE] WHERE ColumnB NOT IN (4,6)
AND ColumnG not in
(SELECT ColumnG
FROM
(
SELECT ColumnG,ColumH,ColumnI FROM [DB].[SCHEMA].[TABLE] EXCEPT
SELECT ColumnG,ColumnH,ColumnI FROM [DB].[SCHEMA].[VIEW]
WHERE VIEW.ColumnB='Active' and year(LastChgDateTime) = 9999
) AAA )
Thanks for any help!

Answer 1

Without knowing your schema and indexing, it's hard to say. We also haven;t seen a query plan. And you haven't provided the view definition, so we don't know what's involved with that.
But for a start, you can simplify this query in the following way
Note the use of a sarge-able predicate on LastChgDateTime
DELETE FROM [DB].[SCHEMA].[TABLE]
WHERE ColumnB NOT IN (4,6)
AND NOT EXISTS (
SELECT ColumnG,ColumH,ColumnI
FROM [DB].[SCHEMA].[TABLE] AAA
WHERE AAA.ColumnG = [TABLE].ColumnG
EXCEPT
SELECT ColumnG,ColumnH,ColumnI
FROM [DB].[SCHEMA].[VIEW]
WHERE [VIEW].ColumnB = 'Active' and LastChgDateTime >= '9999-01-01'
);
For the above, the following indexes would make sense
The view would need indexing on the base tables
[TABLE] (ColumnG, ColumH, ColumnI) INCLUDE (ColumnB)
[VIEW] (ColumnB, ColumnG, ColumH, ColumnI, LastChgDateTime)
We can optimize this further by using an updatable CTE with a window function.
I'm not entirely sure the logic you are trying to achieve, but it appears to be something like this.
WITH cte AS (
SELECT
t.*,
IsGNotInView = COUNT(v.IsNotInView) OVER (PARTITION BY t.ColumnG)
FROM [DB].[SCHEMA].[TABLE] t
CROSS APPLY (
SELECT
CASE WHEN NOT EXISTS (SELECT 1
FROM [DB].[SCHEMA].[VIEW] v
WHERE v.ColumnB = 'Active'
AND v.LastChgDateTime >= '9999-01-01'
AND v.ColumnG = t.ColumnG
AND v.ColumnH = t.ColumnH
AND t.ColumnI = v.ColumnI
)
THEN 1 END
) v(IsNotInView)
)
DELETE FROM cte
WHERE ColumnB NOT IN (4,6)
AND IsGNotInView = 0;