[IMP] fields: add a primary key for m2m tables

Remove individual m2m indexes and replace with a single mirrored
composite

This will help for logical replication strategies (postgresql BDR)

closes odoo/odoo#37963

Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>

odoo/fields.py

@@ -3141,10 +3141,9 @@ class Many2many(_RelationalMulti):
             query = """
                 CREATE TABLE "{rel}" ("{id1}" INTEGER NOT NULL,
                                       "{id2}" INTEGER NOT NULL,
-                                      UNIQUE("{id1}","{id2}"));
+                                      PRIMARY KEY("{id1}","{id2}"));
                 COMMENT ON TABLE "{rel}" IS %s;
-                CREATE INDEX ON "{rel}" ("{id1}");
-                CREATE INDEX ON "{rel}" ("{id2}")
+                CREATE INDEX ON "{rel}" ("{id2}","{id1}");
             """.format(rel=self.relation, id1=self.column1, id2=self.column2)
             cr.execute(query, ['RELATION BETWEEN %s AND %s' % (model._table, comodel._table)])
             _schema.debug("Create table %r: m2m relation between %r and %r", self.relation, model._table, comodel._table)