On Mon, Jun 22, 2020 at 4:53 PM Andres Freund <and...@anarazel.de> wrote: > > Or maybe we add 1 or 2 "privileged" built-in compressors that get > > dedicated bit-patterns in the upper 2 bits of the size field, with the > > last bit pattern being reserved for future algorithms. (e.g. 0x00 = > > pglz, 0x01 = lz4, 0x10 = zstd, 0x11 = something else - see within for > > details). > > Agreed. I favor an approach roughly like I'd implemented below > https://postgr.es/m/20130605150144.GD28067%40alap2.anarazel.de > I.e. leave the vartag etc as-is, but utilize the fact that pglz > compressed datums starts with a 4 byte length header, and that due to > the 1GB limit, the first two bits currently have to be 0. That allows to > indicate 2 compression methods without any space overhead, and > additional compression methods are supported by using an additional byte > (or some variable length encoded larger amount) if both bits are 1.
I think there's essentially no difference between these two ideas, unless the two bits we're talking about stealing are not the same in the two cases. Am I missing something? > One additional note: Adding additional vartag_external values does incur > some noticable cost, distributed across lots of places. OK. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
