Answer by Hannah Vernon for How can I improve a slow comparison query that have over partition and group by

Since [ExternalTable] is linked to a view on another database server, it probably makes sense to "materialize" the rows in [ExternalTable] into a local temp table, then run the query against that.

So, something like this:

CREATE TABLE #t_combined(    SourceTable char(1) NOT NULL    , foo int NOT NULL    , bar int NOT NULL    , [data] int NOT NULL    , INDEX ix CLUSTERED (foo, bar, [data], SourceTable));INSERT INTO #t_combined WITH (TABLOCKX) (SourceTable, foo, bar, [data])SELECT '2', foo, bar, [data]FROM [ExternalTable] et;INSERT INTO #t_combined WITH (TABLOCKX) (SourceTable, foo, bar, [data])SELECT '1', foo, bar, [data]FROM [Table] et;;WITH CombinedTables AS (    SELECT SourceTable        , foo        , bar        , data        , ROW_NUMBER() OVER (PARTITION BY foo, bar, data ORDER BY foo, bar, data) Row_Nbr    FROM #t_combined    GROUP BY foo        , bar        , data        , SourceTable)SELECT DISTINCT * FROM CombinedTables WHERE CombinedTables.Row_Nbr < 2     AND CombinedTables.SourceTable = '2';