How to write sophisticated subquery as from clause in ecto? - postgresql

Question

I'm trying to write in Ecto syntax following sql query, how to write subquery after FROM hierarchy, line, it's in from clause, but I doubt if it is possible in Ecto? I wonder if I can perform such query with use of table joins or even lateral joins without performance loss with the same effect?
SELECT routes.id, routes.name
FROM routes
WHERE routes.id IN
(SELECT DISTINCT hierarchy.parent
FROM hierarchy,
(SELECT DISTINCT unnest(segments.rels) AS rel
FROM segments
WHERE ST_Intersects(segments.geom, ST_SetSrid(ST_MakeBox2D(ST_GeomFromText('POINT(1866349.262143 6886808.978425)', -1), ST_GeomFromText('POINT(1883318.282423 6876413.542579)', -1)), 3857))) AS anon_1
WHERE hierarchy.child = anon_1.rel)
I've stuck on following code:
hierarchy_subquery =
Hierarchy
|> distinct([h], h.parent)
Route
|> select([r], r.id, r.name)
|> where([r], r.id in subquery(hierarchy_subquery))
Schemas:
defmodule MyApp.Hierarchy do
use MyApp.Schema
schema "hierarchy" do
field :parent, :integer
field :child, :integer
field :deph, :integer
end
end
defmodule MyApp.Route do
use MyApp.Schema
schema "routes" do
field :name, :string
field :intnames, :map
field :symbol, :string
field :country, :string
field :network, :string
field :level, :integer
field :top, :boolean
field :geom, Geo.Geometry, srid: 3857
end
end
defmodule MyApp.Segment do
use MyApp.Schema
schema "segments" do
field :ways, {:array, :integer}
field :nodes, {:array, :integer}
field :rels, {:array, :integer}
field :geom, Geo.LineString, srid: 3857
end
end
EDIT I've tested performance of various queries and this below is fastest:
from r in Route,
join: h in Hierarchy, on: r.id == h.parent,
join: s in subquery(
from s in Segment,
distinct: true,
where: fragment("ST_Intersects(?, ST_SetSrid(ST_MakeBox2D(ST_GeomFromText('POINT(1285982.015631 7217169.814674)', -1), ST_GeomFromText('POINT(2371999.313507 6454022.524275)', -1)), 3857))", s.geom),
select: %{rel: fragment("unnest(?)", s.rels)}
),
where: s.rel == h.child,
select: {r.id, r.name}
Results:
Planning time: ~0.605 ms Execution time: ~37.232 ms
The same query as above but join replaced by inner_lateral_join for segments subquery:
Planning time: ~1.353 ms Execution time: ~38.518 ms
Subqueries from answer:
Planning time: ~1.017 ms Execution time: ~41.288 ms
I thought that inner_lateral_join would be faster but it isn't. Does anybody know how to speed up this query?

Answer 1

Here is what I would try. I haven't verified it works but it should point to the proper direction:
segments =
from s in Segment,
where: fragment("ST_Intersects(?, ST_SetSrid(ST_MakeBox2D(ST_GeomFromText('POINT(1866349.262143 6886808.978425)', -1), ST_GeomFromText('POINT(1883318.282423 6876413.542579)', -1)), 3857)))", s.geom),
distinct: true,
select: %{rel: fragment("unnest(?)", s.rel)}
hierarchy =
from h in Hierarchy,
join: s in subquery(segments),
where: h.child == s.rel,
distinct: true,
select: %{parent: h.parent}
routes =
from r in Route,
join: h in subquery(hierarchy),
where: r.top and r.id == h.parent
Things to keep in mind:
Start from the inner query and go to the outer one
To access the result of a subquery, you need to select a map in the subquery
Ecto only allows subqueries in from and join. The good news is that you can usually rewrite "x IN subquery" as a join
You can try to run each query individually and see if they work