Inflate sampled results in Stats.Funnel.funnel/3 (#3270)

* Inflate sampled results in Stats.Funnel.funnel/3

* Add test for sampling

* Update test/plausible/funnels_test.exs

Co-authored-by: hq1 <hq@mtod.org>

* Ignore unused binding

* Use bang version of hash function

---------

Co-authored-by: hq1 <hq@mtod.org>

lib/plausible/stats/funnel.ex

@@ -10,6 +10,7 @@ defmodule Plausible.Stats.Funnel do
   alias Plausible.Funnels
 
   import Ecto.Query
+  import Plausible.Stats.Fragments
 
   alias Plausible.ClickhouseRepo
   alias Plausible.Stats.Base
@@ -60,7 +61,7 @@ defmodule Plausible.Stats.Funnel do
   defp query_funnel(query, funnel_definition) do
     q_events =
       from(e in query,
-        select: %{user_id: e.user_id},
+        select: %{user_id: e.user_id, _sample_factor: fragment("any(_sample_factor)")},
         where: e.site_id == ^funnel_definition.site_id,
         group_by: e.user_id,
         order_by: [desc: fragment("step")]
@@ -69,7 +70,7 @@ defmodule Plausible.Stats.Funnel do
 
     query =
       from(f in subquery(q_events),
-        select: {f.step, count(1)},
+        select: {f.step, total()},
         group_by: f.step
       )
 

test/plausible/funnels_test.exs

@@ -321,5 +321,35 @@ defmodule Plausible.FunnelsTest do
                 ]
               }} = funnel_data
     end
+
+    @tag :slow
+    test "sampling", %{site: site, steps: [g1, g2, g3 | _]} do
+      {:ok, funnel} =
+        Funnels.create(
+          site,
+          "From blog to signup and purchase",
+          [g1, g2, g3]
+        )
+
+      stats =
+        1..50_000
+        |> Enum.flat_map(fn _ ->
+          user_id = SipHash.hash!("0123456789ABCDEF", :crypto.strong_rand_bytes(64))
+
+          [
+            build(:pageview, pathname: "/go/to/blog/foo", user_id: user_id),
+            build(:event, name: "Signup", user_id: user_id),
+            build(:pageview, pathname: "/checkout", user_id: user_id)
+          ]
+        end)
+
+      populate_stats(site, stats)
+
+      query =
+        Plausible.Stats.Query.from(site, %{"period" => "all", "sample_threshold" => "10000"})
+
+      {:ok, funnel_data} = Stats.funnel(site, query, funnel.id)
+      assert_in_delta funnel_data[:all_visitors], 50_000, 1000
+    end
   end
 end