Data access using decompression maps

A computer program product for decompressing data in a database system, the computer program product comprising a computer readable non-transitory storage medium having program code embodied therewith, the program code being executable by a processor to:

• receive, by the processor, a query pertaining to a subset of data within a compressed set of data, wherein the subset of data includes one of:
• a subset of columns in a row of a database table, a subset of columns or bytes within a row of the database table, and a subset of rows within a page of the database table;
• in response to receiving the query, evaluate, by the processor, two or more potential decompression strategies using a cost model, the cost model including an estimated filter factor;
• select, by the processor, a low cost decompression strategy based on the results of the evaluation of the two or more decompression strategies;
• locate, by the processor, one or more bytes representing the requested subset of data within the compressed set of data, wherein the locating includes:
• determining, by the processor, a unit representing a certain number of bytes of compressed data;
• and
• determining, by the processor, what units need to be decompressed to get to the one or more bytes representing the requested subset of data within the compressed set of data;
• decompress, by the processor, only a portion of the compressed data that corresponds to the subset of data, using the selected decompression strategy, while leaving the remaining compressed set of data in an unchanged state;
• and
• store, by the processor, a map for each row, the map describing offsets to the start of every unit, wherein the map is stored as an array in a memory buffer of the database system.