We propose a new way of indexing XML documents and processing twig patterns in an XML database. Every XML document in the database can be transformed into a sequence of labels by Pr?fer's method that constructs a one-to-one correspondence between trees and sequences. During query processing, a twig pattern is also transformed into its Pr?fer sequence. By performing subsequence matching on the set of sequences in the database, and performing a series of refinement phases that we have developed, we can find all the occurrences of a twig pattern in the database. Our approach allows holistic processing of a twig pattern without breaking the twig into root-to-leaf paths and processing these paths individually. Furthermore, we show in the paper that all correct answers are found without any false dismissals or false alarms. Experimental results demonstrate the performance benefits of our proposed techniques.