pools: add post_ids, is_deleted, category indexes.

db/migrate/20190908035317_add_indexes_to_pools.rb

@@ -0,0 +1,7 @@
+class AddIndexesToPools < ActiveRecord::Migration[6.0]
+  def change
+    add_index :pools, :category
+    add_index :pools, :is_deleted
+    add_index :pools, :post_ids, using: :gin
+  end
+end