On Thu, Nov 23, 2023 at 1:10 PM Nikhil Benesch <nikhil.bene...@gmail.com> wrote: > > While working on Materialize's streaming logical replication from Postgres > [0], > my colleagues Sean Loiselle and Petros Angelatos (CC'd) discovered today what > appears to be a correctness bug in pgoutput, introduced in v15. > > The problem goes like this. A table with REPLICA IDENTITY FULL and some > data in it... > > CREATE TABLE t (a int); > ALTER TABLE t REPLICA IDENTITY FULL; > INSERT INTO t VALUES (1), (2), (3), ...; > > ...undergoes a schema change to add a new column with a default: > > ALTER TABLE t ADD COLUMN b bool DEFAULT false NOT NULL; > > PostgreSQL is smart and does not rewrite the entire table during the schema > change. Instead it updates the tuple description to indicate to future readers > of the table that if `b` is missing, it should be filled in with the default > value, `false`. > > Unfortunately, since v15, pgoutput mishandles missing attributes. If a > downstream server is subscribed to changes from t via the pgoutput plugin, > when > a row with a missing attribute is updated, e.g.: > > UPDATE t SET a = 2 WHERE a = 1 > > pgoutput will incorrectly report b's value as NULL in the old tuple, rather > than > false. >
Thanks, I could reproduce this behavior. I'll look into your patch. > Using the same example: > > old: a=1, b=NULL > new: a=2, b=true > > The subscriber will ignore the update (as it has no row with values > a=1, b=NULL), and thus the subscriber's copy of `t` will become out of sync > with > the publisher's. > > I bisected the problem to 52e4f0cd4 [1], which introduced row filtering for > publications. The problem appears to be the use of CreateTupleDescCopy where > CreateTupleDescCopyConstr is required, as the former drops the constraints > in the tuple description (specifically, the default value constraint) on the > floor. > > I've attached a patch which both fixes the issue and includes a test. I've > verified that the test fails against the current master and passes against > the patched version. > > I'm relatively unfamiliar with the project norms here, but assuming the patch > is > acceptable, this strikes me as important enough to warrant a backport to both > v15 and v16. > Right. -- With Regards, Amit Kapila.
