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/jobs/create_recent_post_search_indexes_spec.rb

@@ -0,0 +1,45 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe Jobs::CreateRecentPostSearchIndexes do
+  subject { described_class.new }
+
+  fab!(:post) do
+    SearchIndexer.enable
+    Fabricate(:post)
+  end
+
+  fab!(:post_2) do
+    SearchIndexer.enable
+    Fabricate(:post)
+  end
+
+  before do
+    SearchIndexer.enable
+  end
+
+  describe '#execute' do
+    it 'should not create the index if requried posts size has not been reached' do
+      SiteSetting.search_recent_posts_size = 1
+      SiteSetting.search_enable_recent_regular_posts_offset_size = 3
+
+      expect do
+        subject.execute({})
+      end.to_not change { SiteSetting.search_recent_regular_posts_offset_post_id }
+    end
+
+    it 'should create the right index' do
+      SiteSetting.search_recent_posts_size = 1
+      SiteSetting.search_enable_recent_regular_posts_offset_size = 1
+
+      subject.execute({})
+
+      expect(SiteSetting.search_recent_regular_posts_offset_post_id).to eq(post_2.id)
+
+      expect(DB.query_single(<<~SQL).first).to eq(1)
+      SELECT 1 FROM pg_indexes WHERE indexname = '#{described_class::REGULAR_POST_SEARCH_DATA_INDEX_NAME}'
+      SQL
+    end
+  end
+end