I have a few instances of DB2 10.5 server running on one physical Linux machine, let name them INST1 and INST2.
All of them contain multiple schemas, however schema-naming is unique accross whole machine, for example INST1_SCHEMA_A, INST2_SCHEMA_A etc.
What I would like to do is to somehow create a user that can access all of those schemas as they were on one instance, so it would be possible to make a queries like: SELECT ID FROM INST1_SCHEMA_A UNION SELECT ID FROM INST2_SCHEMA_A
How can I achieve that? Should I just link databases and alias schemas?
Federation
is the keyword for your request. DB2 LUW to DB2 LUW is included in the license and this could be done across multiple databases - not matter if they reside within the same instance, another instance on the same server or even a different server.
Set FEDERATED = YES in the DBM CFG, define a server and set up Nicknames for remote tables. For details refer to this article or this one or the IBM Knowledge Center.