Thanks Justin. Do you know if Postgres treats an UPDATE that sets the indexed columns set to the same previous values as a change? Or does it only count it as "changed" if the values are different. This is ambiguous to me.
*> HOT solves this problem for a restricted but useful special case where a tuple is repeatedly updated in ways that do not change its indexed columns.* *> With HOT, a new tuple placed on the same page and with all indexed columns the same as its parent row version does not get new index entries.* *> [HOT] will create a new physical heap tuple when inserting, and not a new index tuple, if and only if the update did not affect indexed columns.* On Thu, Nov 22, 2018 at 2:40 PM Justin Pryzby <pry...@telsasoft.com> wrote: > On Thu, Nov 22, 2018 at 01:31:10PM -0800, Abi Noda wrote: > > In other words, is Postgres smart enough to not actually write to disk > any > > columns that haven’t changed value or update indexes based on those > columns? > > You're asking about what's referred to as Heap only tuples: > > https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/access/heap/README.HOT;hb=HEAD > https://wiki.postgresql.org/wiki/Index-only_scans#Interaction_with_HOT > > Note, if you're doing alot of updates, you should consider setting a lower > the > table fillfactor, since HOT is only possible if the new tuple (row > version) is > on the same page as the old tuple. > > |With HOT, a new tuple placed on the same page and with all indexed > columns the > |same as its parent row version does not get new index entries." > > And check pg_stat_user_tables to verify that's working as intended. > > Justin >
