Using ALL/filesort on UNION - mysql

Question

so I have this:
EXPLAIN (SELECT f.prn AS 'prn', f.id AS 'xnyz', 'f' AS 'omnf', f.wwn, (COUNT(fr.rid) > 0 OR COUNT(frr.rid) > 0) AS 'rrr', fr.rid AS 'rid', f.xyzz
FROM scmf f
LEFT JOIN scmr fr
ON fr.omnf = 'f' AND fr.xnyz = f.id
LEFT JOIN scmr frr
ON frr.fid = f.id
WHERE f.prn IN (0,43570,43571,43572,43573,43574,43575) AND f.xyzz IN (490923) AND f.id IN (0,43570,43571,43572,43573,43574,43575)
GROUP BY f.id)
UNION
(SELECT fi.fxyz AS 'prn', fi.iix AS 'xnyz', 'n' AS 'omnf', fi.wwn,(COUNT(fir.rid) > 0) AS 'rrr', 0 AS 'rid', fif.xyzz
FROM scmf_item fi
LEFT JOIN scmr fir ON fir.omnf = 'n' AND fir.xnyz = fi.iix
LEFT JOIN scmf fif ON fif.id = fi.fxyz
WHERE fi.fxyz IN (0,43570,43571,43572,43573,43574,43575) AND fif.xyzz IN (490923)
GROUP BY fi.iix
)
ORDER BY wwn ASC;
And an explain reveals:
1 PRIMARY f range PRIMARY,xyzz,prn PRIMARY 4 7 Using where
1 PRIMARY fr ref omnf_id omnf_id 9 const,s.f.id 1 Using index
1 PRIMARY frr ref Index1 Index1 4 s.f.id 1 Using index
2 UNION fi range fxyz fxyz 4 24 Using where; Using temporary; Using filesort
2 UNION fif eq_ref PRIMARY,xyzz PRIMARY 4 s.fi.fxyz 1 Using where
2 UNION fir ref omnf_id omnf_id 9 const,s.fi.iix 1 Using index
UNION RESULT <union1,2> ALL Using filesort
Why is the UNION clause using filesort/have type as ALL
is this bad?
Can I use indices to fix this?