7. A document retrieval method executed by a computer which comprises a document storage unit for storing a document to be searched, the method comprising: storing in a memory a data size of the document such that the data size is associated with a document ID for identifying the document, reading out from the memory the data size associated with the document ID indicating the document to be searched, and calculating a retrieval document size by adding up the read out data size(s), calculating a first estimated time taken for a retrieval process in accordance with a first retrieval method and a second estimated time taken for the retrieval process in accordance with a second retrieval method, based on the retrieval document size, and comparing the first and second estimated times, and deciding which retrieval method to use for performing the retrieval process, the first method or the second method, wherein the computer comprises an index storage unit for storing a retrieval key and the document ID indicating the document containing the retrieval key such that they are associated with each other; the first retrieval method is an index scan method of retrieving the document ID associated with the retrieval key from the index storage unit; and the second retrieval method is a text scan method of searching, for each of the documents stored in the document storage unit, whether or not the retrieval key is contained in the document, and the document retrieval method further comprising receiving an input of first and second retrieval keys, retrieving the document containing the first retrieval key in accordance with the first retrieval method, storing in the memory the document ID indicating the retrieved document containing the first retrieval key, reading out from the memory the data size associated with each of the retrieved document IDs stored in the memory, calculating the retrieval document size by adding up the read out data sizes, and comparing the first and second estimated times, and deciding which retrieval method to use for performing the process of retrieving the document containing the second retrieval key, the first retrieval method or the second retrieval method.
7. A document retrieval method executed by a computer which comprises a document storage unit for storing a document to be searched, the method comprising: storing in a memory a data size of the document such that the data size is associated with a document ID for identifying the document, reading out from the memory the data size associated with the document ID indicating the document to be searched, and calculating a retrieval document size by adding up the read out data size(s), calculating a first estimated time taken for a retrieval process in accordance with a first retrieval method and a second estimated time taken for the retrieval process in accordance with a second retrieval method, based on the retrieval document size, and comparing the first and second estimated times, and deciding which retrieval method to use for performing the retrieval process, the first method or the second method, wherein the computer comprises an index storage unit for storing a retrieval key and the document ID indicating the document containing the retrieval key such that they are associated with each other; the first retrieval method is an index scan method of retrieving the document ID associated with the retrieval key from the index storage unit; and the second retrieval method is a text scan method of searching, for each of the documents stored in the document storage unit, whether or not the retrieval key is contained in the document, and the document retrieval method further comprising receiving an input of first and second retrieval keys, retrieving the document containing the first retrieval key in accordance with the first retrieval method, storing in the memory the document ID indicating the retrieved document containing the first retrieval key, reading out from the memory the data size associated with each of the retrieved document IDs stored in the memory, calculating the retrieval document size by adding up the read out data sizes, and comparing the first and second estimated times, and deciding which retrieval method to use for performing the process of retrieving the document containing the second retrieval key, the first retrieval method or the second retrieval method. 9. The document retrieval method according to claim 7 , wherein: the index storage unit stores index information in which the retrieval key and the document ID are associated with each other, and the document retrieval method further comprising storing in the memory an index size that is the total amount of data sizes of all of the index information related to the retrieval key, such that the index size is associated with the retrieval key, storing in the memory a document read-out amount per unit time, which is the data amount that can be read out from the document storage unit per the unit time, and an index read-out amount per unit time, which is the data amount that can be read out from the index storage unit per the unit time, calculating as the second estimated time a time required for reading out all of the documents associated with the retrieved document IDs from the document storage unit, based on the retrieval document size and the document read-out amount per unit time, and reading out the index size associated with the second retrieval key from the index size storage unit, and calculating as the first estimated time a time required for reading out all of the indices related to the retrieval key from the index storage unit, based on the read out index size and the index read-out amount per unit time.