Data processing methods and systems for accessing a target sample in a media data stream. The media data stream has a plurality of samples and corresponding sample information recorded in at least one entry, wherein the sample information for a predetermined number of the entries is calculated and the calculated result is stored in at least one cache entry. Target sample information corresponding to the target sample is provided. One of the cache entries is located by comparing the target sample information with the calculated result of respective cache entries. After locating the cache entry, the target sample from the entries is located corresponding to the located cache entry.