performance issue while using select single query

hi all,

While using the select single statment performance getting slow

SELECT SINGLE NODENAME

INTO W_CUST

FROM /BI0/HCUSTOMER

WHERE NODEID = W_CUSTOMER-PARENTID.

in where condition given field is secondary key(PARENTID)

This table Technical setting given below

1. data class --customer data class

2. size category --4(expected data record 13.000 to 520.000

but at present my table number of entries (1.544.092)

3. buffering switched on

4. buffer type generic ( no.of key fields 2)

note my tabel has 10 fields in it 3 primary key fields and one secondary fields(PARENTID),

please give me solution to increase the performance

reward asured

with regards,

Thambe