StackOverflow2013

Note that there are some explanatory texts on larger screens.

plurals

PO
text
Body
copied!<p>Compression is all about removing redundancy from the data. Unfortunately, it's unlikely that the redundancy is going to be distributed with monotonous evenness throughout the file, and that's about the only scenario in which you could expect compression <em>and</em> fine-grained random access. </p> <p>However, you could get <em>close</em> to random access by maintaining an external list, built during the compression, which shows the correspondence between chosen points in the uncompressed datastream and their locations in the compressed datastream. You'd obviously have to choose a method where the translation scheme between the source stream and its compressed version does not vary with the location in the stream (i.e. no LZ77 or LZ78; instead you'd probably want to go for Huffman or byte-pair encoding.) Obviously this would incur a lot of overhead, and you'd have to decide on just how you wanted to trade off between the storage space needed for "bookmark points" and the processor time needed to decompress the stream starting at a bookmark point to get the data you're actually looking for on that read.</p> <p>As for random-access writing... that's all but impossible. As already noted, compression is about removing redundancy from the data. If you try to replace data that could be and was compressed because it was redundant with data that does not <em>have</em> the same redundancy, it's simply not going to fit.</p> <p>However, depending on <em>how much</em> random-access writing you're going to do -- you may be able to simulate it by maintaining a sparse matrix representing all data written to the file after the compression. On all reads, you'd check the matrix to see if you were reading an area that you had written to after the compression. If not, then you'd go to the compressed file for the data.</p>

Querying!

Guidance

An individual column

Larger individual text columns get their own page to allow for proper reading.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload