Wrap part of on_conflict_target in parentheses

[Huliiiiii]

@tisonkun mentioned to me that in PostgreSQL, some statements used as the index_expression in a conflict_target need to be wrapped in parentheses.

https://www.postgresql.org/docs/current/sql-insert.html

where conflict_target can be one of:

    ( { index_column_name | ( index_expression ) } [ COLLATE collation ] [ opclass ] [, ...] ) [ WHERE index_predicate ]
    ON CONSTRAINT constraint_name