Fix returning more pageviews with a visit property filter (#2612)

* fix subquery for sessions in base_event_query/2

As the 'sessions' table is using the CollapsingMergeTree engine, we have
to select session_id's distinctively. Otherwise we will get multiple rows
(with sign -1 and 1) as long as the background merge hasn't happened.

* update changelog

* use GROUP BY instead of SELECT DISTINCT

* remove comma

CHANGELOG.md

@@ -7,6 +7,7 @@ All notable changes to this project will be documented in this file.
 - 'Last updated X seconds ago' info to 'current visitors' tooltips
 
 ### Fixed
+- Fix [more pageviews with session prop filter than with no filters](https://github.com/plausible/analytics/issues/1666)
 - Cascade delete sent_renewal_notifications table when user is deleted plausible/analytics#2549
 - Show appropriate top-stat metric labels on the realtime dashboard when filtering by a goal
 - Fix breakdown API pagination when using event metrics plausible/analytics#2562

lib/plausible/stats/base.ex

@@ -14,7 +14,9 @@ defmodule Plausible.Stats.Base do
       sessions_q =
         from(
           s in query_sessions(site, query),
-          select: %{session_id: s.session_id}
+          select: %{session_id: s.session_id},
+          where: s.sign == 1,
+          group_by: s.session_id
         )
 
       from(

test/plausible_web/controllers/api/external_stats_controller/aggregate_test.exs

@@ -798,5 +798,31 @@ defmodule PlausibleWeb.Api.ExternalStatsController.AggregateTest do
 
       assert json_response(conn, 200)["results"] == %{"visitors" => %{"value" => 3}}
     end
+
+    test "joins correctly with the sessions (CollapsingMergeTree) table", %{
+      conn: conn,
+      site: site
+    } do
+      create_sessions([
+        %{domain: site.domain, session_id: 1000, country_code: "EE", sign: 1, events: 1},
+        %{domain: site.domain, session_id: 1000, country_code: "EE", sign: -1, events: 1},
+        %{domain: site.domain, session_id: 1000, country_code: "EE", sign: 1, events: 2}
+      ])
+
+      create_events([
+        %{domain: site.domain, session_id: 1000, country_code: "EE", name: "pageview"},
+        %{domain: site.domain, session_id: 1000, country_code: "EE", name: "pageview"},
+        %{domain: site.domain, session_id: 1000, country_code: "EE", name: "pageview"}
+      ])
+
+      conn =
+        get(conn, "/api/v1/stats/aggregate", %{
+          "site_id" => site.domain,
+          "metrics" => "pageviews",
+          "filters" => "visit:country==EE"
+        })
+
+      assert json_response(conn, 200)["results"] == %{"pageviews" => %{"value" => 3}}
+    end
   end
 end