Performance improvements to comments search (#13489)

* Explicitly pluck user_id from comments table

* Change comment query

* Remove indexes and add partial indexes

* Update schema

app/services/search/postgres/comment.rb

@@ -23,13 +23,6 @@ module Search
       ].freeze
       private_constant :USER_ATTRIBUTES
 
-      ARTICLE_COMMENTABLE_QUERY = <<-SQL.freeze
-        LEFT JOIN articles
-          ON comments.commentable_id = articles.id
-          AND comments.commentable_type = 'Article'
-      SQL
-      private_constant :ARTICLE_COMMENTABLE_QUERY
-
       DEFAULT_PER_PAGE = 60
       private_constant :DEFAULT_PER_PAGE
 
@@ -42,19 +35,6 @@ module Search
       MAX_PER_PAGE = 120 # to avoid querying too many items, we set a maximum amount for a page
       private_constant :MAX_PER_PAGE
 
-      # We filter comments for those that are:
-      # 1. On Articles
-      # 2. Not deleted
-      # 3. Not hidden by commentable user (i.e. an Article author didn't hide the comment)
-      # 4. Are attached to published articles
-      QUERY_FILTER = <<-SQL.freeze
-        comments.commentable_type = 'Article' AND
-        comments.deleted = false AND
-        comments.hidden_by_commentable_user = false AND
-        articles.published = true
-      SQL
-      private_constant :QUERY_FILTER
-
       def self.search_documents(
         page: 0,
         per_page: DEFAULT_PER_PAGE,
@@ -74,7 +54,14 @@ module Search
         page = page.to_i + 1
         per_page = [(per_page || DEFAULT_PER_PAGE).to_i, MAX_PER_PAGE].min
 
-        relation = ::Comment.joins(ARTICLE_COMMENTABLE_QUERY).where(QUERY_FILTER)
+        relation = ::Comment
+          .where(
+            deleted: false,
+            hidden_by_commentable_user: false,
+            commentable_type: "Article",
+          )
+          .joins("join articles on articles.id = comments.commentable_id")
+          .where("articles.published": true)
 
         relation = relation.search_comments(term).with_pg_search_highlight if term.present?
 
@@ -92,7 +79,7 @@ module Search
         # (see https://github.com/forem/forem/pull/4744#discussion_r345698674
         # and https://github.com/rails/rails/issues/15185#issuecomment-351868335
         # for additional context)
-        user_ids = results.pluck(:user_id)
+        user_ids = results.pluck("comments.user_id")
         users = find_users(user_ids)
 
         serialize(results, users)

db/migrate/20210423162805_remove_indexes_from_comments.rb

@@ -0,0 +1,15 @@
+class RemoveIndexesFromComments < ActiveRecord::Migration[6.1]
+  disable_ddl_transaction!
+
+  def up
+    remove_index :comments, column: :commentable_type, algorithm: :concurrently if index_exists?(:comments, :commentable_type)
+    remove_index :comments, column: :deleted, algorithm: :concurrently if index_exists?(:comments, :deleted)
+    remove_index :comments, column: :hidden_by_commentable_user, algorithm: :concurrently if index_exists?(:comments, :hidden_by_commentable_user)
+  end
+
+  def down
+    add_index :comments, :commentable_type, algorithm: :concurrently unless index_exists?(:comments, :commentable_type)
+    add_index :comments, :deleted, algorithm: :concurrently unless index_exists?(:comments, :deleted)
+    add_index :comments, :hidden_by_commentable_user, algorithm: :concurrently unless index_exists?(:comments, :hidden_by_commentable_user)
+  end
+end

db/migrate/20210423162847_add_partial_indexes_from_comments.rb

@@ -0,0 +1,13 @@
+class AddPartialIndexesFromComments < ActiveRecord::Migration[6.1]
+  disable_ddl_transaction!
+
+  def up
+    add_index :comments, :deleted, where: "deleted = false", algorithm: :concurrently unless index_exists?(:comments, :deleted)
+    add_index :comments, :hidden_by_commentable_user,where: "hidden_by_commentable_user = false",  algorithm: :concurrently unless index_exists?(:comments, :hidden_by_commentable_user)
+  end
+
+  def down
+    remove_index :comments, column: :deleted, algorithm: :concurrently if index_exists?(:comments, :deleted)
+    remove_index :comments, column: :hidden_by_commentable_user, algorithm: :concurrently if index_exists?(:comments, :hidden_by_commentable_user)
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2021_04_22_171642) do
+ActiveRecord::Schema.define(version: 2021_04_23_162847) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "citext"
@@ -392,10 +392,9 @@ ActiveRecord::Schema.define(version: 2021_04_22_171642) do
     t.index ["ancestry"], name: "index_comments_on_ancestry"
     t.index ["ancestry"], name: "index_comments_on_ancestry_trgm", opclass: :gin_trgm_ops, using: :gin
     t.index ["commentable_id", "commentable_type"], name: "index_comments_on_commentable_id_and_commentable_type"
-    t.index ["commentable_type"], name: "index_comments_on_commentable_type"
     t.index ["created_at"], name: "index_comments_on_created_at"
-    t.index ["deleted"], name: "index_comments_on_deleted"
-    t.index ["hidden_by_commentable_user"], name: "index_comments_on_hidden_by_commentable_user"
+    t.index ["deleted"], name: "index_comments_on_deleted", where: "(deleted = false)"
+    t.index ["hidden_by_commentable_user"], name: "index_comments_on_hidden_by_commentable_user", where: "(hidden_by_commentable_user = false)"
     t.index ["score"], name: "index_comments_on_score"
     t.index ["user_id"], name: "index_comments_on_user_id"
   end