Lookup table for a one-to-many relationship
Your table will work fine for this purpose, but you probably want to add an index. If the primary reason for using this table is to take an outside_ticket_id and get the corresponding ticket_id's I would add the following clustered index:

CREATE CLUSTERED INDEX [CL_Lookup_OD_ID] on [lookup](outside_data_id)
GO


If the primary lookup will be the other way around (trying to find the outside_data_id  from a ticket_id) place the clustered index on the other column.

----  Oh, sorry, just noticed this is Postgres. The above syntax is SQL Server.  For Postgres, create an index on the column, then issue the cluster command, like so:

CREATE INDEX IX_outside_data_id on lookup(outside_data_id);
CLUSTER lookup using IX_outside_data_id;


You may also want to look at the "fillfactor" on the index depending on how heavy the insert load is here.  But that is a large topic worth exploring on it's own...