--- a/cubicweb/misc/migration/3.23.0_Any.py	Thu Oct 20 18:45:40 2016 +0200
+++ b/cubicweb/misc/migration/3.23.0_Any.py	Thu Oct 20 18:28:46 2016 +0200
@@ -11,6 +11,24 @@
 source = repo.system_source
 helper = source.dbhelper
 
+# drop all relations primary keys
+for table, cstr in sql('''
+    SELECT DISTINCT tc.table_name, tc.constraint_name
+    FROM information_schema.table_constraints tc,
+         information_schema.key_column_usage kc
+    WHERE tc.constraint_type IN 'PRIMARY KEY'
+          AND kc.table_name = tc.table_name
+          AND kc.table_name LIKE '%\_relation'
+          AND kc.table_schema = tc.table_schema
+          AND kc.constraint_name = tc.constraint_name;
+'''):
+    sql('ALTER TABLE %s DROP CONSTRAINT' % (table, cstr))
+
+for table, cstr in sql("""
+    SELECT DISTINCT table_name, constraint_name FROM information_schema.constraint_column_usage
+    WHERE table_name LIKE 'cw\_%' AND constraint_name LIKE '%\_key'"""):
+    sql("ALTER TABLE %(table)s DROP CONSTRAINT %(cstr)s" % locals())
+
 for rschema in schema.relations():
     if rschema.rule or rschema in PURE_VIRTUAL_RTYPES:
         continue
@@ -20,7 +38,7 @@
             column = 'cw_{0}'.format(rdef.rtype)
             if any(isinstance(cstr, UniqueConstraint) for cstr in rdef.constraints):
                 old_name = '%s_%s_key' % (table.lower(), column.lower())
-                sql('ALTER TABLE %s DROP CONSTRAINT IF EXISTS %s' % (table, old_name))
+                sql('ALTER TABLE %s DROP CONSTRAINT %s' % (table, old_name))
                 source.create_index(cnx, table, column, unique=True)
             if rschema.inlined or rdef.indexed:
                 old_name = '%s_%s_idx' % (table.lower(), column.lower())
@@ -28,7 +46,6 @@
                 source.create_index(cnx, table, column)
     else:
         table = '{0}_relation'.format(rschema)
-        sql('ALTER TABLE %s DROP CONSTRAINT IF EXISTS %s_p_key' % (table, table))
         sql('ALTER TABLE %s ADD CONSTRAINT %s PRIMARY KEY(eid_from, eid_to)'
             % (table, build_index_name(table, ['eid_from', 'eid_to'], 'key_')))
         for column in ('from', 'to'):
@@ -36,13 +53,12 @@
             sql('CREATE INDEX %s ON %s(eid_%s);'
                 % (build_index_name(table, ['eid_' + column], 'idx_'), table, column))
 
-
 # we changed constraint serialization, which also changes their name
 
 for table, cstr in sql("""
-    SELECT table_name, constraint_name FROM information_schema.constraint_column_usage
+    SELECT DISTINCT table_name, constraint_name FROM information_schema.constraint_column_usage
     WHERE constraint_name LIKE 'cstr%'"""):
-    sql("ALTER TABLE %(table)s DROP CONSTRAINT IF EXISTS %(cstr)s" % locals())
+    sql("ALTER TABLE %(table)s DROP CONSTRAINT %(cstr)s" % locals())
 
 for cwconstraint in rql('Any C WHERE R constrained_by C').entities():
     cwrdef = cwconstraint.reverse_constrained_by[0]