PERF: Create a partial regular post_search_data index on large sites.

With the addition of `PostSearchData#private_message`, a partial
index consisting of only search data from regular posts can be created.
The partial index helps to speed up searches on large sites since PG
will not have to do an index scan on the entire search data index which
has shown to be a bottle neck.

spec/components/search_spec.rb

@@ -543,27 +543,24 @@ describe Search do
       ])
     end
 
-    it "allows the configuration of search to prefer recent posts" do
-      SiteSetting.search_prefer_recent_posts = true
-      SiteSetting.search_recent_posts_size = 1
-      post = Fabricate(:post, topic: topic, raw: "this is a play post")
+    it "is able to search with an offset when configured" do
+      post_1 = Fabricate(:post, raw: "this is a play post")
+      SiteSetting.search_recent_regular_posts_offset_post_id = post_1.id + 1
+
+      results = Search.execute('play post')
+
+      expect(results.posts).to eq([post_1])
+
+      post_2 = Fabricate(:post, raw: "this is another play post")
+
+      SiteSetting.search_recent_regular_posts_offset_post_id = post_2.id
 
       results = Search.execute('play post')
 
       expect(results.posts.map(&:id)).to eq([
-        post.id
+        post_2.id,
+        post_1.id
       ])
-
-      post2 = Fabricate(:post, raw: "this is a play post")
-
-      results = Search.execute('play post')
-
-      expect(results.posts.map(&:id)).to eq([
-        post2.id,
-        post.id
-      ])
-    ensure
-      Discourse.cache.clear
     end
 
     it 'allows staff to search for whispers' do