Adding SELECT COUNT(*) subclause trashes performance in SQL Server - sql

Question

I'm building a query and the latest step involved adding a SELECT COUNT(*) FROM [modification] sub-clause to allow me to detect the last row of the query, but it destroys the performance:
SELECT CONCAT(
IIF(row_number() OVER (ORDER BY forecastId) % 50000 = 1,
CONCAT('INSERT INTO modification (userId, epochTime, ',
'forecastId, description, auxText, auxDate) VALUES ('),
' ('),
userId, ',',
epochTime, ',',
forecastId, ',',
'''', dbo.encode4MySql(description), ''',',
'''', dbo.encode4MySql(auxText), ''',',
'''', CONVERT(VARCHAR(20), auxDate, 120), ''')',
IIF(row_number() OVER (ORDER BY forecastId) % 50000 = 0
OR row_number() OVER (ORDER BY forecastId) =
(SELECT COUNT(*) FROM modification),
'; COMMIT;', ','))
FROM modification
ORDER BY forecastId;
If you can't see what I'm doing, I'm building INSERT () VALUES (),(),(),... statements of 50000 rows at a time.
Please restrict suggestions for completely alternative approaches to the comments. I'm looking for a way to find the last row number here without it slowing the query down massively - which it does.
I'm not massively familiar with query plans but can post one here if it helps. I've tried a lot of things from related questions here, but nothing that I can get to work.

Answer 1

One other option would be to order by forecastId desc in the final or:
IIF(row_number() OVER (ORDER BY forecastId) % 50000 = 0
OR row_number() OVER (ORDER BY forecastId desc) = 1,
'; COMMIT;', ','))

Answer 2

You have a rather complicated expression, so SQL Server may not optimize it. Move the logic to the FROM clause:
FROM (SELECT m.*, COUNT(*) OVER () as num_rows
FROM modification m
) m
And then use num_rows in the rest of the query.

Answer 3

Assuming you don't want to change the current design, you could just add an extra UNION ALL step at the end. By looking at your query, it looks like the only purpose of changing the query is to add a COMMIT at the end.
CURRENT QUERY
UNION ALL
SELECT 'COMMIT;';
Let me know if that works for you.
***********UPDATE*********
I thought this query is easier to troubleshoot. See if it will perform any better. You would have to plugin the CTE part for your table.
SELECT BusinessEntityID,JobTitle,HireDate INTO dbo.TestTable FROM [HumanResources].[Employee]
SELECT TOP 0 BusinessEntityID,JobTitle,HireDate INTO dbo.TestTable2 FROM [HumanResources].[Employee]
SET NOCOUNT ON
WITH CTE AS (SELECT ROW_NUMBER() OVER(ORDER BY (SELECT NULL)) Col,
' (' +
CAST(BusinessEntityID AS VARCHAR(20)) +
', ''' +
JobTitle +
''', ''' +
CONVERT(VARCHAR(20), HireDate, 120) +
''')' Query
FROM TestTable
)
SELECT
CASE
WHEN COL % 50 = 0 THEN ', ' + Query + ' COMMIT;'
WHEN COL % 50 = 1 THEN ' INSERT INTO dbo.TestTable2 (BusinessEntityID, JobTitle, HireDate) VALUES ' + Query
ELSE ', ' + Query
END
FROM CTE
UNION ALL SELECT 'COMMIT;'