Re: error: could not find pg_class tuple for index 2662

Ahh ... you know what, never mind about stack traces, let's just see if
the attached patch doesn't fix it.

I still haven't reproduced the behavior here, but I think I see what
must be happening: we are getting an sinval reset while attempting to
open pg_class_oid_index.  The latter condition causes its refcount to
be above 1, which will cause RelationClearRelation to directly call
RelationReloadIndexInfo, which enables the following sequence of calls:
RelationCacheInvalidate -> RelationClearRelation -> RelationReloadIndexInfo.
And the problem is that RelationCacheInvalidate intentionally forces
pg_class_oid_index to be updated first.  That was okay when the code was
written, because the relcache entry for pg_class itself never really
needed any updates.  Now it does, so we have to make sure pg_class gets
updated first, *then* pg_class_oid_index (which might result in a
seqscan of pg_class), then everything else (for which we'll try to use
pg_class_oid_index to locate their pg_class tuples).

            regards, tom lane

diff --git a/src/backend/utils/cache/relcache.c b/src/backend/utils/cache/relcache.c
index 81cea8b..0e4b17c 100644
*** a/src/backend/utils/cache/relcache.c
--- b/src/backend/utils/cache/relcache.c
*************** RelationCacheInvalidate(void)
*** 2185,2204 ****
          {
              /*
               * Add this entry to list of stuff to rebuild in second pass.
!              * pg_class_oid_index goes on the front of rebuildFirstList, other
!              * nailed indexes on the back, and everything else into
!              * rebuildList (in no particular order).
               */
!             if (relation->rd_isnailed &&
!                 relation->rd_rel->relkind == RELKIND_INDEX)
              {
                  if (RelationGetRelid(relation) == ClassOidIndexId)
-                     rebuildFirstList = lcons(relation, rebuildFirstList);
-                 else
                      rebuildFirstList = lappend(rebuildFirstList, relation);
              }
              else
!                 rebuildList = lcons(relation, rebuildList);
          }
      }

--- 2185,2207 ----
          {
              /*
               * Add this entry to list of stuff to rebuild in second pass.
!              * pg_class goes on the front of rebuildFirstList,
!              * pg_class_oid_index goes to the back of rebuildFirstList, other
!              * nailed indexes go on the front of rebuildList, and everything
!              * else goes to the back of rebuildList.
               */
!             if (RelationGetRelid(relation) == RelationRelationId)
!                 rebuildFirstList = lcons(relation, rebuildFirstList);
!             else if (relation->rd_isnailed &&
!                      relation->rd_rel->relkind == RELKIND_INDEX)
              {
                  if (RelationGetRelid(relation) == ClassOidIndexId)
                      rebuildFirstList = lappend(rebuildFirstList, relation);
+                 else
+                     rebuildList = lcons(relation, rebuildList);
              }
              else
!                 rebuildList = lappend(rebuildList, relation);
          }
      }