ORA-1555 SNAPSHORT TO OLD ERROR POSSIBLE CUASE ARE..
A.UNDO SEGMENT IS SMALL
B.UNDO RETANTION IS LOW VALUE
C.USER NOT COMMIT THERE TRANSACTION IMMEDIATLY.
SO SOLUTION IS ...
1.INCREASE THE SIZE OF UNDO TABLESPACE
2.USE SPECIFIC ROLLBACK SEGMENT FOR LARGE TRANSACTION BASICALY BATCH PROCESS.
3.MAKE UNDO_SUPPRESS_ERROR=TRUE

BEST OF LUCK
LIAKAT HOSSAIN
MCP,OCAAD9I,OCPDBA8I,9I,10G,OCA10gAS.