PostgreSQL: renaming an attribute of a dj.U restriction quotes it with backticks #1638

Summary

On PostgreSQL, renaming an attribute in a projection of a dj.U restriction produces invalid SQL. (dj.U("b") & T).proj(c="b") compiles to SELECT DISTINCT `b` as "c" ...: the renamed attribute is quoted with MySQL backticks, which PostgreSQL rejects. MySQL returns the right rows.

The cause appears to be Heading.set_primary_key, which builds the new heading without table_info. The projection then quotes the attribute without the connection's adapter, and falls back to backticks.

Reproduction

DataJoint master, PostgreSQL 15:

import datajoint as dj

schema = dj.Schema("scratch_u_proj")


@schema
class T(dj.Manual):
    definition = """
    a : int32
    ---
    b : int32
    """


T.insert([(1, 10), (2, 10), (3, 20)])
(dj.U("b") & T).proj(c="b").to_arrays("c")
# QuerySyntaxError: syntax error at or near "as"
# LINE 1: SELECT DISTINCT `b` as "c" FROM "scratch_u_proj"."t"

On MySQL the same query returns [10, 20].

Expected behavior

The projection quotes every attribute for the connection's backend, and returns [10, 20] on PostgreSQL too.