Currently, when you use the Add Incrementing ID Field geoprocessing tool to add an OBJECTID field to a non-geodatabase maintained spatial table, the associated unique index created by the tool ends up in whatever the database sees as the default tablespace. At least in PostgreSQL that I am using, I see the index ending up in the pg_default tablespace.
It would be useful to be able to optionally specify the tablespace where the index will be created, so as to be able to avoid the use of the default tablespace.