This is very slow:
select F_KEY, count(*) as P1FM_F_KEY_C from ( select FILE_KEY as F_KEY from NPFP1.NPF_FILE_MIGRATION where FILE_KEY IS NOT NULL union all select FILE_ERROR_KEY as F_KEY from NPFP1.NPF_FILE_MIGRATION WHERE FILE_ERROR_KEY IS NOT NULL ) FK GROUP By F_KEY
This is much faster
select F_KEY, sum(P1FM_F_KEY_C) as P1FM_F_KEY_C from ( select FILE_KEY as F_KEY, count(*) as P1FM_F_KEY_C from NPFP1.NPF_FILE_MIGRATION group by FILE_KEY union all select FILE_ERROR_KEY as F_KEY, count(*) as P1FM_F_KEY_C from NPFP1.NPF_FILE_MIGRATION group by FILE_ERROR_KEY ) T group by F_KEY