On Sat, Nov 21, 2020 at 12:58 AM Andy Fan <zhihui.fan1...@gmail.com> wrote: > I don't mean we need to be the same as Oracle, but to support a > customer who comes from Oracle, it would be good to know the > difference.
Actually, it is documented here: https://www.postgresql.org/docs/devel/transaction-iso.html The description starts with: "UPDATE, DELETE, SELECT FOR UPDATE, and SELECT FOR SHARE commands behave the same as SELECT in terms of searching for target rows...". I imagine that the number of application developers that are aware of this specific aspect of transaction isolation in PostgreSQL (READ COMMITTED conflict handling/EvalPlanQual()) is extremely small. In practice it doesn't come up that often. Though Postgres hackers tend to think about it a lot because it is hard to maintain. I'm not saying that that's good or bad. Just that that has been my experience. I am sure that some application developers really do understand the single most important thing about READ COMMITTED mode's behavior: each new command gets its own MVCC snapshot. But I believe that Oracle is no different. So in practice application developers probably don't notice any difference between READ COMMITTED mode in practically all cases. (Again, just my opinion.) -- Peter Geoghegan
