Views/tables based on catalog views can cause issues during metadata sync

[onderkalaci]

Citus automatically propagates views that are created on catalog tables/view:

SET citus.log_remote_commands TO ON;
create view v1 as select * from information_schema.table_constraints;
....
NOTICE:  issuing CREATE OR REPLACE VIEW public.v1 (constraint_catalog,constraint_schema,constraint_name,table_catalog,table_schema,table_name,constraint_type,is_deferrable,initially_deferred,enforced,nulls_distinct) AS  SELECT table_constraints.constraint_catalog,
...

This makes sense, because the user can also rename the information_schema.table_constraints, such as:

ALTER VIEW information_schema.table_constraints RENAME TO another_view;

Now, the v1 still works from the workers and coordinator as well.

But, the caveat here is the metadata sync. Say, you upgrade from Citus 9.5 to Citus 11.3. And, you first upgraded the workers to PG 15. And, while the coordinator is still on PG 14, you run CALL citus_finish_citus_upgrade().

In that scenario, Citus would try to re-create the information_schema.table_constraints. That is mostly fine. But, given that PG 15 has an additional column compared to PG 14, Postgres complained:

ERROR:  cannot drop columns from view

We had to upgrade the coordinator to PG 15, then re-run CALL citus_finish_citus_upgrade().

[onderkalaci]

citus_finish_citus_upgrade checks whether the Citus versions match. We should expand it to check PG versions as well.

[vlkncetin]

While upgrading from PG11 to 14, and Citus 9.5 to 12.1, CALL citus_finish_citus_upgrade(); query just gave the following error:

NOTICE:  Preparing to sync the metadata to all nodes
WARNING:  column pg_stat_statements.total_time does not exist
ERROR:  failure on connection marked as essential: private-w0.myhostname
CONTEXT:  SQL statement "SELECT start_metadata_sync_to_all_nodes()"
PL/pgSQL function citus_finalize_upgrade_to_citus11(boolean) line 174 at PERFORM
SQL statement "SELECT citus_finalize_upgrade_to_citus11()"  
PL/pgSQL function citus_finish_citus_upgrade() line 25 at PERFORM   

PG11 has pg_stat_statements.total_time, but PG14 does not.