1. A computer-implemented method for dynamically querying structured and unstructured data repositories, the method comprising: maintaining, by a computer, a unified index associated with a structured data repository and an unstructured data repository; receiving, by the computer, a unified query of keywords including a first keyword for querying the structured data repository and a second keyword for querying the unstructured data repository; first querying, by the computer, one of the structured data repository and the unstructured data repository; if first querying the structured data repository, then, using the computer, matching the unified index to first querying results with the first keyword, and using unified index matches to limit second querying of the unstructured data repository with the second keyword; and if first querying the unstructured data repository, then, using the computer, matching the unified index to first querying results with the second keyword, and using unified index matches to limit second querying of the structured data repository with the first keyword, wherein the first querying of the one of the structured data repository and the unstructured data repository is decided at query time using occurrence statistics for the keywords, thereby selecting an efficient approach to answer a query.