Hi ChandraShekar,
If your database has up-to-date index structures, try this:
for $PSTNCallData in (collection("52_DATA_AUGUST-14_Billed_HugeDataTesting")/SUBCUSTBRK/ITEMIZED/TRANSACTION[@Type = 'PSTN' ]/CALL_TYPES/DATA/R) return <R><bill_no>{$PSTNCallData/C1}</bill_no></R>
Please give me some feedback if this solves your problems. If not, please send me the query info output.
Best, Christian