From ac80360bea2ce4c9e93a38395d16e2ecc2cee2ed Mon Sep 17 00:00:00 2001
From: Guo Xiang Tan <tgx_world@hotmail.com>
Date: Thu, 21 Jun 2018 13:26:26 +0800
Subject: [PATCH] PERF: Help postgres make use of index in `Post.summary`.

---
 app/models/post.rb | 23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/app/models/post.rb b/app/models/post.rb
index f37a9cf1510..5e736977101 100644
--- a/app/models/post.rb
+++ b/app/models/post.rb
@@ -349,13 +349,22 @@ class Post < ActiveRecord::Base
 
     # percent rank has tons of ties
     where(topic_id: topic_id)
-      .where(["(post_number = 1) or id in (
-          SELECT p1.id
-          FROM posts p1
-          WHERE p1.percent_rank <= ? AND
-             p1.topic_id = #{topic_id}
-          ORDER BY p1.percent_rank
-          LIMIT ?
+      .where([
+        "id = ANY(
+          (
+            SELECT posts.id
+            FROM posts
+            WHERE posts.topic_id = #{topic_id.to_i}
+            AND post_number = 1
+          ) UNION
+          (
+            SELECT p1.id
+            FROM posts p1
+            WHERE p1.percent_rank <= ?
+            AND p1.topic_id = #{topic_id.to_i}
+            ORDER BY p1.percent_rank
+            LIMIT ?
+          )
         )",
         SiteSetting.summary_percent_filter.to_f / 100.0,
         SiteSetting.summary_max_results