… segfault #7705 (#7718)
This PR is a proposed fix for issue
[7705](https://github.com/citusdata/citus/issues/7705). The following is
the background and rationale for the fix (please refer to
[7705](https://github.com/citusdata/citus/issues/7705) for context);
The `varnullingrels `field was introduced to the Var node struct
definition in Postgres 16. Its purpose is to associate a variable with
the set of outer join relations that can cause the variable to be NULL.
The `varnullingrels ` for the variable
`"gianluca_camp_test"."start_timestamp"` in the problem query is 3,
because the variable "gianluca_camp_test"."start_timestamp" is coming
from the inner (nullable) side of an outer join and 3 is the RT index
(aka relid) of that outer join. The problem occurs when the Postgres
planner attempts to plan the combine query. The format of a combine
query is:
```
SELECT <targets>
FROM pg_catalog.citus_extradata_container();
```
There is only one relation in a combine query, so no outer joins are
present, but the non-empty `varnullingrels `field causes the Postgres
planner to access structures for a non-existent relation. The source of
the problem is that, when creating the target list for the combine
query, function MasterAggregateMutator() uses copyObject() to construct
a Var node before setting the master table ID, and this copies over the
non-empty varnullingrels field in the case of the
`"gianluca_camp_test"."start_timestamp"` var. The proposed solution is
to have MasterAggregateMutator() use makeVar() instead of copyObject(),
and only set the fields that make sense for the combine query; var type,
collation and type modifier. The `varnullingrels `field can be left
empty because there is only one relation in the combine query.
A new regress test issue_7705.sql is added to exercise the fix. The
issue is not specific to window functions, any target expression that
cannot be pushed down and contains at least one column from the inner
side of a left outer join (so has a non-empty varnullingrels field) can
cause the same issue.
More about Citus combine queries
[here](https://github.com/citusdata/citus/tree/main/src/backend/distributed#combine-query-planner).
More about Postgres varnullingrels
[here](https://github.com/postgres/postgres/blob/master/src/backend/optimizer/README).
(cherry picked from commit 248ff5d52a)
DESCRIPTION: PR description that will go into the change log, up to 78
characters
Co-authored-by: Colm <colm.mchugh@gmail.com>
Emel Şimşek