performance issue for rseg table

hi,

we have an fi report related to inter state tax calculation.

the report is woking fine.

problem is here we r selecting data from rseg based on mseg

but we cannot use any primary key combination.so the database is taking long time to retrive data.

i checked in both se30 and st05.the below statement is taking more time to retrive data from database.

IF NOT it_mseg1[] IS INITIAL.

SELECT belnr gjahr buzei lfbnr lfgja lfpos ebeln ebelp FROM rseg

INTO TABLE it_rseg

FOR ALL ENTRIES IN it_mseg1

WHERE lfbnr = it_mseg1-mblnr AND

lfgja = it_mseg1-mjahr AND

lfpos = it_mseg1-zeile.

ENDIF.

can any one suggest in improving the performance .