Permit outer joins when using update_all with PostreSQL

[frerich]

The PostgreSQL UPDATE command permits specifying multiple 'from_item' expressions to allow columns from other tables to appear in the update expression. In particular, the documentation explains (cf. https://www.postgresql.org/docs/current/sql-update.html):

This uses the same syntax as the FROM clause of a SELECT statement

Thus, a statement such as

UPDATE students student
SET
  name = person.name,
  tutor_name = tutor.name
FROM persons person
LEFT JOIN tutors tutor ON tutor.id = person.tutor_id 
WHERE student.person_id = person.id

is legal.

However, executing query like this this via Ecto would fail, raising an error:

  ** (Ecto.QueryError) PostgreSQL supports only inner joins on update_all, got: `left` in query:

  from x0 in "x",
    join: y1 in "y",
    on: true,
    left_join: z2 in "z",
    on: true,
    update: [set: [a: 1]]

This PR fixes the issue by adding a Postgres.Connection.using_join/4 clause for the :update_all case: in this scenario, at least one inner join has to be specified, but additional other joins are permissible.

[josevalim]

@frerich perhaps, instead of doing the validation ourselves, what if we simply allow the user to specify the joins and we emit the query? Will Postgres show a proper error message telling them they need at least one inner join or the error message is not good?

[frerich]

what if we simply allow the user to specify the joins and we emit the query? Will Postgres show a proper error message telling them they need at least one inner join or the error message is not good?

@josevalim PostgreSQL merely reports a syntax error:

ERROR:  syntax error at or near "LEFT"
LINE 5: LEFT JOIN tutors tutor ON tutor.id = person.tutor_id

That didn't seem terribly friendly to me. 😔

The PostgreSQL adapter already seems to make some effort to come up with helpful hints as to why something goes wrong (e.g. "the :conflict_target option is required on upserts by PostgreSQL") and since this check seemed easy enough, I felt it would be plausible to keep it.

[josevalim]

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