求帮忙优化一下SQL
SELECT ltrim(rtrim(ta_main8.c_dpt_cde)),COALESCE(CHAR(TA_MAINC.N_CLM_COUNT),'0'),0,0,
sum(1),
sum(ta_main8.n_prm_var_yuan),
0, 0, 0, 0,0, 0, 'A'
FROM rpt_plyedr TA_MAIN8 LEFT OUTER JOIN (SELECT C_PLY_NO,COUNT(DISTINCT c_clm_no) N_CLM_COUNT FROM RPT_CLM_MAIN WHERE C_CNL_MRK = 'Z' GROUP BY C_PLY_NO) TA_MAINC ON TA_MAINC.C_PLY_NO =TA_MAIN8.C_PLY_NO
WHERE TA_MAIN8.t_insrnc_end_tmBETWEEN '2015-09-23-00.00.00.000000'AND'2015-09-23-23.59.59.999999'
and ta_main8.c_edr_type = '0'
and (ta_main8.c_ply_status not in ('2','3')
or (ta_main8.c_ply_statusin ('2','3') and ta_main8.t_end_tm > '2015-09-23-23.59.59.999999'))
and ta_main8.c_kind_no not in ( '03')
AND TA_MAIN8.C_PROD_NO BETWEEN '0101' AND '2650'AND TA_MAIN8.GN_INTER_CDE LIKE '99%'
GROUP BY ltrim(rtrim(ta_main8.c_dpt_cde)),COALESCE(CHAR(TA_MAINC.N_CLM_COUNT),'0');
求助：怎么改写，避免对（SELECT C_PLY_NO,COUNT(DISTINCT c_clm_no) N_CLM_COUNT FROM RPT_CLM_MAIN WHERE C_CNL_MRK = 'Z' GROUP BY C_PLY_NO）的全表扫描排序？？
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