FEATURE: An option to search more recent posts for very large sites.

On very large forums searching posts can be slow, so this commit
introduces the ability to try and search only the most recent posts
first, and then going for a larger breadth search if there aren't
enough results.

Enable `search_prefer_recent_posts` and you can customize how many
recent posts to filter with `search_recent_posts_size`

lib/search.rb

@@ -128,6 +128,24 @@ class Search
     end
   end
 
+  def self.min_post_id_no_cache
+    return 0 unless SiteSetting.search_prefer_recent_posts?
+
+    offset = Post.count - SiteSetting.search_recent_posts_size
+    return 0 if offset <= 0
+
+    Post.order(:id).offset(offset).limit(1).pluck(:id)[0]
+  end
+
+  def self.min_post_id(opts=nil)
+    return 0 unless SiteSetting.search_prefer_recent_posts?
+
+    # It can be quite slow to count all the posts so let's cache it
+    Rails.cache.fetch("search-min-post-id:#{SiteSetting.search_recent_posts_size}", expires_in: 1.day) do
+      min_post_id_no_cache
+    end
+  end
+
   attr_accessor :term
 
   def initialize(term, opts=nil)
@@ -545,7 +563,16 @@ class Search
           posts = posts.joins('JOIN users u ON u.id = posts.user_id')
           posts = posts.where("posts.raw  || ' ' || u.username || ' ' || COALESCE(u.name, '') ilike ?", "%#{term_without_quote}%")
         else
+
+
           posts = posts.where("post_search_data.search_data @@ #{ts_query}")
+
+          min_id = Search.min_post_id
+          if min_id > 0
+            fast_query = posts.dup.where("post_search_data.post_id >= #{min_id}")
+            posts = fast_query if fast_query.dup.count >= 20
+          end
+
           exact_terms = @term.scan(/"([^"]+)"/).flatten
           exact_terms.each do |exact|
             posts = posts.where("posts.raw ilike ?", "%#{exact}%")
@@ -669,7 +696,7 @@ class Search
             .to_sql
         else
           posts_query(@limit, aggregate_search: true,
-                                     private_messages: opts[:private_messages])
+                              private_messages: opts[:private_messages])
             .select('topics.id', "#{min_or_max}(post_number) post_number")
             .group('topics.id')
             .to_sql