posts: add string_to_array(tag_string, ' ') index.

This is preparation for removing tag_index and test_parser.

db/migrate/20211010181657_add_string_to_array_tag_string_index_on_posts.rb

@@ -0,0 +1,20 @@
+class AddStringToArrayTagStringIndexOnPosts < ActiveRecord::Migration[6.1]
+  disable_ddl_transaction!
+
+  def change
+    add_index :posts, "string_to_array(tag_string, ' ')", using: :gin, algorithm: :concurrently
+
+    up_only do
+      # Set the statistics target on the index to 3000 so that Postgres will a)
+      # collect stats on the size of top 3000 largest tags and b) sample
+      # 3000*300 = 900k random posts to do so. This is necessary so that
+      # Postgres can generate good query plans based on the size of the tag.
+      #
+      # https://akorotkov.github.io/blog/2017/05/31/alter-index-weird/
+      # https://www.postgresql.org/docs/current/planner-stats.html
+      # https://www.postgresql.org/docs/current/sql-alterindex.html
+      execute "ALTER INDEX index_posts_on_string_to_array_tag_string ALTER COLUMN 1 SET STATISTICS 3000"
+      execute "VACUUM (VERBOSE, ANALYZE) posts"
+    end
+  end
+end