1. A method for building an index structure for a relational database table, comprising: obtaining at least one JavaScript Object Notation (JSON) document from the relational database table; identifying a plurality of JSON objects in the at least one JSON document and assigning identifiers for the plurality of JSON objects; and building the index structure according to values in the plurality of JSON objects such that the index structure includes at least one node which records the values in one of the plurality of JSON objects and includes a corresponding identifier of the one of the plurality of JSON objects.