Add a GIN index on comments.ancestry (#13120)

We have a btree index on this table but it is not invoked with a `LIKE`
operation that uses `%` - only when using `LIKE` with an exact match.

This query is DEV's second-most intense query by total time spent with
a p50 latency of ~130ms. This index brings it down to 5ms at p90.

db/migrate/20210325040245_add_trigram_index_to_comments_ancestry.rb

@@ -0,0 +1,14 @@
+class AddTrigramIndexToCommentsAncestry < ActiveRecord::Migration[6.0]
+  disable_ddl_transaction!
+
+  def change
+    add_index :comments, :ancestry,
+      # There is already a btree index on this column, so we need to specify a different name
+      name: 'index_comments_on_ancestry_trgm',
+      # Using trigram operations requires a GIN index
+      using: :gin,
+      # Indexing for LIKE operations requires trigram operations
+      opclass: :gin_trgm_ops,
+      algorithm: :concurrently
+  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_03_24_031738) do
+ActiveRecord::Schema.define(version: 2021_03_25_040245) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "citext"
@@ -387,6 +387,7 @@ ActiveRecord::Schema.define(version: 2021_03_24_031738) do
     t.bigint "user_id"
     t.index "digest(body_markdown, 'sha512'::text), user_id, ancestry, commentable_id, commentable_type", name: "index_comments_on_body_markdown_user_ancestry_commentable", unique: true
     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 ["created_at"], name: "index_comments_on_created_at"
     t.index ["score"], name: "index_comments_on_score"