4. The method of claim 1 , wherein the identifying the plurality of portions within the respective document comprises: traversing, by the computing device, at least one word in the forward index of the respective document; recording, by the computing device, the at least one traversed word and a location of the at least one traversed word in a first data structure by using the location of the traversed word as an index of the first data structure; and when the at least one traversed word matches one of the keywords, recording, by the computing device, the at least one traversed word and the location of the at least one traversed word in a second data structure by using the at least one traversed word as an index of the second data structure.
4. The method of claim 1 , wherein the identifying the plurality of portions within the respective document comprises: traversing, by the computing device, at least one word in the forward index of the respective document; recording, by the computing device, the at least one traversed word and a location of the at least one traversed word in a first data structure by using the location of the traversed word as an index of the first data structure; and when the at least one traversed word matches one of the keywords, recording, by the computing device, the at least one traversed word and the location of the at least one traversed word in a second data structure by using the at least one traversed word as an index of the second data structure. 19. The method of claim 4 , wherein the finding the portion among the plurality of portions comprises: determining, by the computing device, the beginning position of the portion and the ending position of the portion according to the first data structure, the second data structure, and the length limit, the determining including: in response to determining that a total length of traversed words in the first data structure is less than the length limit, setting the beginning position to a location of a first traversed word in the first data structure and setting the ending position to a location of a last traversed word in the first data structure; or in response to determining that the total length of traversed words in the first data structure is not less than the length limit, utilizing the second data structure to determine one or more portions of the respective document that have lengths less than the length limit, to determine the portion of the one or more portions that has the highest number of the one or more keywords, and to set the beginning position and ending position of the portion based on the locations of a first traversed word and a last traversed word, respectively, in the portion.