search: refactor fast_count to return nil on timeout.

* Refactor fast_count to return nil instead of 1,000,000 if the exact count times out.
* Remove the estimate_post_counts and blank_tag_search_fast_count global config options.
* Replace the hardcoded post count estimates inside fast_count with a
  method that parses Postgres's estimated row count from EXPLAIN.

* /counts/posts.json:
** Remove the `raise_on_timeout` parameter.
** Add an `estimate_count=<true|false>` parameter.
** Return null instead of 1,000,000 if the exact count times out.

app/controllers/counts_controller.rb

@@ -2,6 +2,8 @@ class CountsController < ApplicationController
   respond_to :xml, :json
 
   def posts
-    @count = PostQueryBuilder.new(params[:tags], CurrentUser.user).fast_count(timeout: CurrentUser.statement_timeout, raise_on_timeout: true, skip_cache: params[:skip_cache])
+    estimate_count = params.fetch(:estimate_count, "true").truthy?
+    skip_cache = params.fetch(:skip_cache, "false").truthy?
+    @count = PostQueryBuilder.new(params[:tags], CurrentUser.user).fast_count(timeout: CurrentUser.statement_timeout, estimate_count: estimate_count, skip_cache: skip_cache)
   end
 end

app/logical/explain_parser.rb

@@ -0,0 +1,15 @@
+class ExplainParser < Struct.new(:sql)
+  extend Memoist
+
+  def query_plan
+    result = ApplicationRecord.connection.select_one("EXPLAIN (FORMAT JSON) #{sql}")
+    json = JSON.parse(result["QUERY PLAN"])
+    json.first["Plan"]
+  end
+
+  def row_count
+    query_plan["Plan Rows"]
+  end
+
+  memoize :query_plan
+end

app/logical/post_query_builder.rb

@@ -825,81 +825,50 @@ class PostQueryBuilder
   end
 
   concerning :CountMethods do
-    def fast_count(timeout: 1_000, raise_on_timeout: false, skip_cache: false)
-      tags = to_s
-
-      # Optimize some cases. these are just estimates but at these
-      # quantities being off by a few hundred doesn't matter much
-      if Danbooru.config.estimate_post_counts
-        if tags == ""
-          return (Post.maximum(:id).to_i * (2200402.0 / 2232212)).floor
-
-        elsif tags =~ /^rating:s(?:afe)?$/
-          return (Post.maximum(:id).to_i * (1648652.0 / 2200402)).floor
-
-        elsif tags =~ /^rating:q(?:uestionable)?$/
-          return (Post.maximum(:id).to_i * (350101.0 / 2200402)).floor
-
-        elsif tags =~ /^rating:e(?:xplicit)?$/
-          return (Post.maximum(:id).to_i * (201650.0 / 2200402)).floor
-
-        end
-      end
-
+    def fast_count(timeout: 1_000, estimate_count: true, skip_cache: false)
       count = nil
-
-      unless skip_cache
-        count = get_count_from_cache
-      end
-
-      if count.nil?
-        count = fast_count_search(timeout: timeout, raise_on_timeout: raise_on_timeout)
-      end
-
+      count = estimated_count if estimate_count
+      count = cached_count if count.nil? && !skip_cache
+      count = exact_count(timeout) if count.nil?
       count
-    rescue Post::SearchError
-      0
     end
 
-    def fast_count_search(timeout:, raise_on_timeout:)
+    def estimated_count
+      if is_empty_search?
+        estimated_row_count
+      elsif is_simple_tag?
+        Tag.find_by(name: tags.first.name).try(:post_count)
+      elsif is_metatag?(:rating)
+        estimated_row_count
+      end
+    end
+
+    def estimated_row_count
+      ExplainParser.new(build.to_sql).row_count
+    end
+
+    def cached_count
+      Cache.get(count_cache_key)
+    end
+
+    def exact_count(timeout)
       count = Post.with_timeout(timeout, nil) do
         build.count
       end
 
-      if count.nil?
-        # give up
-        if raise_on_timeout
-          raise TimeoutError.new("timed out")
-        end
-
-        count = Danbooru.config.blank_tag_search_fast_count
-      else
-        set_count_in_cache(count)
-      end
-
-      count ? count.to_i : nil
-    rescue PG::ConnectionBad
-      return nil
+      set_cached_count(count) if count.present?
+      count
+    rescue Post::SearchError
+      nil
     end
 
-    def get_count_from_cache
-      if is_simple_tag?
-        count = Tag.find_by(name: tags.first.name).try(:post_count)
-      else
-        # this will only have a value for multi-tag searches or single metatag searches
-        count = Cache.get(count_cache_key)
-      end
-
-      count.try(:to_i)
-    end
-
-    def set_count_in_cache(count)
+    def set_cached_count(count)
       expiry = count.seconds.clamp(3.minutes, 20.hours).to_i
       Cache.put(count_cache_key, count, expiry)
     end
 
     def count_cache_key
-      "pfc:#{Cache.hash(to_s)}"
+      "pfc:#{to_s}"
     end
   end
 

app/logical/post_sets/post.rb

@@ -84,7 +84,7 @@ module PostSets
     def get_post_count
       if %w(json atom xml).include?(format.downcase)
         # no need to get counts for formats that don't use a paginator
-        return Danbooru.config.blank_tag_search_fast_count
+        nil
       else
         query.fast_count
       end
@@ -103,15 +103,11 @@ module PostSets
         if is_random?
           temp = get_random_posts
         else
-          temp = query.build.paginate(page, count: post_count, limit: per_page)
+          temp = query.build.paginate(page, count: post_count, search_count: !post_count.nil?, limit: per_page)
         end
       end
     end
 
-    def unknown_post_count?
-      post_count == Danbooru.config.blank_tag_search_fast_count
-    end
-
     def hide_from_crawler?
       return true if current_page > 1
       return false if query.is_empty_search? || query.is_simple_tag? || query.is_metatag?(:order, :rank)

app/views/counts/posts.html.erb

@@ -4,7 +4,7 @@
 <div id="c-counts">
   <div id="a-posts">
     Post count for <%= link_to params[:tags].present? ? params[:tags] : "/posts", posts_path(:tags => params[:tags]) %>:
-    <% if @count == (Danbooru.config.blank_tag_search_fast_count || 1_000_000) %>
+    <% if @count.nil? %>
       timed out
     <% else %>
       <%= @count %>