On Mon, Sep 30, 2019 at 09:20:22PM +0500, Andrey Borodin wrote:
30 сент. 2019 г., в 20:56, Tomas Vondra <tomas.von...@2ndquadrant.com> написал(а): I mean this: /* * Use int64 to prevent overflow during calculation. */ compressed_size = (int32) ((int64) rawsize * 9 + 8) / 8; I'm not very familiar with pglz internals, but I'm a bit puzzled by this. My first instinct was to compare it to this: #define PGLZ_MAX_OUTPUT(_dlen) ((_dlen) + 4) but clearly that's a very different (much simpler) formula. So why shouldn't pglz_maximum_compressed_size simply use this macro?
compressed_size accounts for possible increase of size during compression. pglz can consume up to 1 control byte for each 8 bytes of data in worst case.
OK, but does that actually translate in to the formula? We essentially need to count 8-byte chunks in raw data, and multiply that by 9. Which gives us something like nchunks = ((rawsize + 7) / 8) * 9; which is not quite what the patch does.
Even if whole data is compressed well - there can be prefix compressed extremely ineffectively. Thus, if you are going to decompress rawsize bytes, you need at most compressed_size bytes of compressed input.
OK, that explains why we can't use the PGLZ_MAX_OUTPUT macro. -- Tomas Vondra http://www.2ndQuadrant.com PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
