select table 'NAST' is very slowly

Dears,

There is a background job running very slowly, Abaper debuged  that found it is very slow when select table 'NAST' in program, detail sentence as below:

SELECT objky
kschl
datvr
uhrvr
usnam
FROM nast
INTO TABLE i_nast
FOR ALL ENTRIES IN i_objk
WHERE objky EQ i_objk-objky
and KSCHL EQ 'ZDOC' .

but in the test system it runs very fast, the NAST table data is 49000000 entries in production system and 44000000 in test system.

we have done the update statistics  for this table and implement SAP Note 1020260 , but performance is still very slow .

Could you please help give any idea ?

Thanks,

Michael