Embodiments are directed to building an index structure for a relational database (RDB) table and for querying the RDB table. An aspect includes obtaining JSON documents from a RDB table, identifying JSON objects in the JSON documents and assigning identifiers for them, and building an index structure according to values in the JSON objects such that the index structure includes at least one node that records values in a JSON object and includes the identifier of the specific JSON object. Querying the RDB table includes receiving a query request including an indicator which indicates that the query is to be performed on JSON objects, and searching the index structure based on query conditions in the query request so as to determine a target JSON object that meets the query conditions.