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, the assigning identifiers for the plurality of JSON objects includes assigning the identifiers based on a relative position of the JSON objects in the at least one JSON document; wherein identifying the plurality of JSON objects in the at least one JSON document comprises identifying indicative symbols in the at least one JSON document, wherein the symbols are in pairs represented in a form of key:value; and wherein a value comprises one or more of an array having a character string within double quotation marks, a number, a Boolean value, an object and another array; combining a document identifier (ID) of the least one JSON document with each of the assigned identifiers of the plurality of JSON objects to create a global identifier (ID) for each of the plurality of JSON objects, wherein the document ID referenced by a corresponding column in the relational database table in which the at least one JSON document is stored; 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 global identifier of the at least one JSON document and the one of the plurality of JSON objects.