On Tue, Jun 18, 2024 at 9:40 PM Amit Kapila <amit.kapil...@gmail.com> wrote: > > On Tue, Jun 18, 2024 at 12:11 PM Peter Smith <smithpb2...@gmail.com> wrote: > > > > While reviewing another thread that proposes to include "generated > > columns" support for logical replication [1] I was looking for any > > existing PostgreSQL documentation on this topic. > > > > But, I found almost nothing about it at all -- I only saw one aside > > mention saying that logical replication low-level message information > > is not sent for generated columns [2]. > > > > ~~ > > > > IMO there should be some high-level place in the docs where the > > behaviour for logical replication w.r.t. generated columns is > > described. > > > > +1. > > > There are lots of candidate places which could talk about this topic. > > * e.g.1 in "Generated Columns" (section 5.4) > > * e.g.2 in LR "Column-Lists" docs (section 29.5) > > * e.g.3 in LR "Restrictions" docs (section 29.7) > > * e.g.4 in the "CREATE PUBLICATION" reference page > > > > For now, I have provided just a simple patch for the "Generated > > Columns" section [3]. Perhaps it is enough. > > > > Can we try to clarify if their corresponding values are replicated? >
Sure. Here are some current PG17 observed behaviours demonstrating that generated columns are not replicated. ====== Example #1 The generated cols 'b' column is not replicated. Notice the subscriber side 'b' has its own computed value which uses a different calculation. PUB: create table t1 (a int, b int generated always as (a * 2) stored); SUB: create table t1 (a int, b int generated always as (a * 20) stored); PUB: insert into t1 values (1),(2),(3); create publication pub1 for table t1; test_pub=# select * from t1; a | b ---+--- 1 | 2 2 | 4 3 | 6 (3 rows) SUB: create subscription sub1 connection 'dbname=test_pub' publication pub1; test_sub=# select * from t1; a | b ---+---- 1 | 20 2 | 40 3 | 60 (3 rows) ====== Example 2 You cannot specify a generated column in a CREATE PUBLICATION column-list. PUB: create table t2 (a int, b int generated always as (a * 2) stored); create publication pub2 for table t2(b); ERROR: cannot use generated column "b" in publication column list ====== Example 3 Here the subscriber-side table doesn't even have a column 'b'. Normally, a missing column like this would cause subscription errors, but since the publisher-side generated column 'b' is not replicated, this scenario is allowed. PUB: create table t3 (a int, b int generated always as (a * 2) stored); SUB: create table t3 (a int); PUB: create publication pub3 for table t3; insert into t3 values (1),(2),(3); test_pub=# select * from t3; a | b ---+--- 1 | 2 2 | 4 3 | 6 (3 rows) SUB: create subscription sub3 connection 'dbname=test_pub' publication pub3; test_sub=# select * from t3; a --- 1 2 3 (3 rows) ====== Kind Regards, Peter Smith. Fujitsu Australia
