artist urls: add trigram indexes on urls.

db/migrate/20180916002448_add_trigram_index_to_artist_urls.rb

@@ -0,0 +1,12 @@
+class AddTrigramIndexToArtistUrls < ActiveRecord::Migration[5.2]
+  def change
+    change_table :artist_urls do |t|
+      t.remove_index column: :url, name: :index_artist_urls_on_url
+      t.remove_index column: :url, name: :index_artist_urls_on_url_pattern, opclass: :text_pattern_ops
+      t.remove_index column: :normalized_url, name: :index_artist_urls_on_normalized_url
+
+      t.index :url, name: :index_artist_urls_on_url_trgm, using: :gin, opclass: :gin_trgm_ops
+      t.index :normalized_url, name: :index_artist_urls_on_normalized_url_trgm, using: :gin, opclass: :gin_trgm_ops
+    end
+  end
+end