Establishing method of database index and computer system thereof

The invention relates to an establishing method of a database index and a computer system thereof. The establishing method comprises the following steps of: obtaining a first index field and a second index field from a relational database when an index establishing command is received, wherein the index establishing command corresponds to a first XML (Extensive Makeup Language) extracting function and an index attribute; then analyzing an XML (Extensive Makeup Language) document by using an internal extracting function according to the characteristics of the first XML extracting function; If the XML (Extensive Makeup Language) document comprises one or a plurality of elements which are in line with the index attribute and are different with each other, recording the one or plurality of elements in the first index field, and correspondingly recording the occurrence number of the one or plurality of elements in the second index field, thereby establishing the index of the XML (Extensive Makeup Language) documen