Andres Freund <and...@anarazel.de> wrote: > On May 30, 2018 9:45:32 AM EDT, Antonin Houska <a...@cybertec.at> wrote: > >Alvaro Herrera <alvhe...@2ndquadrant.com> wrote: > > > >> On 2018-May-30, Antonin Houska wrote: > >> > >> > In the header comment, SnapBuildInitialSnapshot() claims to set > >> > snapshot->satisfies to the HeapTupleSatisfiesMVCC test function, > >and indeed it > >> > converts the "xid" array to match its semantics (i.e. the xid items > >eventually > >> > represent running transactions as opposed to the committed ones). > >However the > >> > test function remains HeapTupleSatisfiesHistoricMVCC as set by > >> > SnapBuildBuildSnapshot(). > >> > >> Interesting. While this sounds like an oversight that should have > >> horrible consequences, it's seems not to because the current callers > >> don't seem to care about the ->satisfies function. Are you able to > >come > >> up with some scenario in which it causes an actual problem? > > > >Right, the current callers in the core do not seem to use that > >function. I hit > >the issue when doing and testing some changes in an extension > >(pg_squeeze). > > What is that extension doing with that snapshot?
It fetches data from a table in order to insert them into a new table, to eliminate bloat. Something like pg_repack, but it uses logical decoding instead of triggers to capture concurrent data changes. -- Antonin Houska Cybertec Schönig & Schönig GmbH Gröhrmühlgasse 26, A-2700 Wiener Neustadt Web: https://www.cybertec-postgresql.com
