Hi, On 2018-03-01 08:17:33 +0300, Arseny Sher wrote: > While prowling through snapbuild & reorderbuffer code, I wondered: why a queue > of snapshots is used for replaying each transaction instead of just picking up > snapshot from snapbuilder once when COMMIT record is read? I am not aware of > any > DDL/DML mix which would make this invalid: e.g. we can't insert something into > table in xact T1, then alter a column in xact T2, and then insert something > more > in T1. All ALTER TABLE forms which are currently interesting from the decoding > POV obtain ACCESS EXCLUSIVE lock, conflicting with any other manipulations on > the > table: > https://www.postgresql.org/docs/devel/static/sql-altertable.html
I don't think that's right. For one there's plenty types of DDL where no such locking exists, consider e.g. composite datums where additional columns can be added even after such a type is already used by another table. For another, T1 might need to see a DDL change to a table that has been made by T2 after T1 started, which is not prohibited by locking if T1 uses the table first after T2 commits. - Andres
