Add support for materialized CTEs

[maartenvanvliet]

Adds support for MATERIALIZED CTE's. Before Postgres 12 CTE's were materialized by default but from then on it was opt-in with the MATERIALIZED keyword. In some cases this can be beneficial because then the CTE can act as a optimisation fence, which can result in better query plans sometimes.

This PR still needs some work, so I'm opening it to see if there's interest in this. If so, have I taken the right approach here?

Corresponding echo_sql PR elixir-ecto/ecto_sql#466

See also https://www.postgresql.org/docs/current/queries-with.html#id-1.5.6.12.7 and https://paquier.xyz/postgresql-2/postgres-12-with-materialize/

[greg-rychlewski]

Something is a bit off, some of the tests that should be there are not in your branch. Like this one in the planner test:

   test "with CTEs" do
      cte_query = from(s in "schema", select: %{x1: selected_as(s.x, :integer), x2: s.x})

      %{with_ctes: %{queries: [{"schema_cte", inner_query}]}} =
        Comment
        |> with_cte("schema_cte", as: ^cte_query)
        |> normalize()

      assert [{:integer, _}, {:x2, _}] = inner_query.select.fields
    end

By any chance is your fork out of date? would you be able to rebase it?

[greg-rychlewski]

Looks good to me thank you. I'll just let @v0idpwn or @josevalim approve as well. Then once this is merged we can point your ecto_sql PR back to it.

[v0idpwn]

LGTM, just 1 question and 1 small fix

[maartenvanvliet]

Thx everyone! Happy holidays! 🎅