FileMetaData.File

    ID INT, Priority SMALLINT, HashSum VARBINARY(MAX)

CREATE INDEX [FileMetaData.File.IX.Priority+Id WHERE HashSum IS NULL] ON [FileMetaData].[File]
(
	[Priority] ASC
)
INCLUDE ([Id]) 
WHERE ([HashSum] IS NULL);

SELECT COUNT(1) FROM FileMetaData.[File]
--WITH (INDEX([FileMetaData.File.IX.Priority+Id WHERE HashSum IS NULL]))
WHERE HashSum IS NULL;

ALTER TABLE FileMetaData.[File] ADD IsEmptyHash AS IIF(HashSum IS NULL, 1, 0)
GO

CREATE INDEX ix2 ON FileMetaData.[File] ([Priority])
INCLUDE ([Id])
WHERE IsEmptyHash = 1
GO

SELECT SUM(p.[rows])
FROM sys.partitions p
JOIN sys.indexes i ON p.[object_id] = i.[object_id]
WHERE p.[object_id] = OBJECT_ID('FileMetaData.[File]')
    AND i.[name] = 'ix2'