The extensions contain an extension index, and observations. The extension index contains all the observation titles of the observations in the extension. The observation title is composed of 32 4-bytes word (see table 1), allowing a quick search for observations matching any combination of these words without having to read all of the file. Observations are independant and can be read individually, which allows only the desired subset of data to be read. With large datasets of hundreds of gigabytes, it is also important to use memory as efficiently as possible so CLIC uses its own cache. The size of the memory that CLIC has access to is controled by the SPACE_CLIC logical name. It can be set with the command SIC LOGICAL SPACE_CLIC value, with value in MB, unless the unit is specified, e.g. 64GB or a fraction of the RAM size is specified, e.g. 50%. The ideal situation is when the value accessible to CLIC is larger than the size of the raw data file (which is another reason to have as compact as possible the files). This is typically the case on the data reduction machines available at IRAM. This reduces the I/O to reading the file once and writing a few calibrations, before producing large uv tables.