fix index on tag name prefixes (#3858)

db/migrate/20180913184128_fix_tag_prefix_index.rb

@@ -0,0 +1,7 @@
+class FixTagPrefixIndex < ActiveRecord::Migration[5.2]
+  def change
+    execute "set statement_timeout = 0"
+    execute "DROP INDEX index_tags_on_name_prefix"
+    execute "CREATE INDEX index_tags_on_name_prefix ON tags USING gin (REGEXP_REPLACE(name, '([a-z0-9])[a-z0-9'']*($|[^a-z0-9'']+)', '\\1', 'g') gin_trgm_ops)"
+  end
+end