switched to a conditional constraint. (#7451)

src/main/resources/db/migration/V5.4.0.1__7451-unique-constraint-storageidentifier.sql

@@ -1 +1,10 @@
-ALTER TABLE dvobject ADD CONSTRAINT unq_dvobject_storageidentifier UNIQUE(owner_id, storageidentifier);
+-- ALTER TABLE dvobject ADD CONSTRAINT unq_dvobject_storageidentifier UNIQUE(owner_id, storageidentifier);
+-- Instead of a uniform constraint on all dvobjects (as above), force a 
+-- conditional unique constraint on datafiles only:
+CREATE UNIQUE INDEX IF NOT EXISTS unq_dvobject_storageidentifier ON dvobject (owner_id, storageidentifier) WHERE dtype='DataFile';
+-- This is not going to have any effect on new databases (since v4.20+), 
+-- where the table was created with the full constraint; but for legacy 
+-- installations it would spare them having to clean up any dataset-level 
+-- storageidentifiers. We know that some old installations have datasets 
+-- with junk values in that column (like "file" - ??) that are meaningless,
+-- but otherwise harmless.