Support DISTINCT ON

DISTINCT ON is a really powerful Postgres feature to perform aggregation-like(pick first in group). It can be used in plain SELECT queries without window function or GROUP BY. It will be great to see support of DISTINCT ON since Hasura has excellent support of ORDER BY.

e.g. We need to get the cheapest fruits in groups by color(see sudo SQL<=>GraphQL below)

SELECT DISTINCT ON
  (color)
  color,
  name,
  price
FROM
  fruit
ORDER BY price;

query {
  fruit(distinct_on: [color], order_by: {price: asc}) {
    color
    name
    price
  }
}

Postgres documentation https://www.postgresql.org/docs/11/sql-select.html#SQL-DISTINCT Usage example https://medium.com/statuscode/the-many-faces-of-distinct-in-postgresql-c52490de5954

[0x777]

@dmoiseenko Can you try this out on this PR build: https://hge-ci-pull-1099.herokuapp.com/

{
  product(distinct_on: color order_by: {color: asc price:desc}) {
    id
    price
    color
  }
}

Note that we force the Postgres restriction that the DISTINCT ON column(s) must match the leftmost ORDER BY column(s).

[rakeshkky]

Hi @cleivson, please note that this issue was related to Postgres. By the time this was closed, we only support single Postgres source (version 1.x). Thank you.