title: Alter Table too slow in postgres
body: <p>I'm trying to add a new column </p>
  
  <pre><code>ALTER TABLE "Cidade" ADD COLUMN "BoundBox" VARCHAR(255)
  </code></pre>
  
  <p>to this table:</p>
  
  <pre><code> "Cidade"
      "Id" integer not null
          constraint "Cidade_PK"
              primary key,
      "Nome" varchar(120),
      "EstadoId" integer not null
          constraint "Estado_Cidade_FK"
              references "Estado",
      "PontoCentralLatitude" numeric,
      "PontoCentralLongitude" numeric
  </code></pre>
  
  <p>But the query never finish, I've already waited for 5 minutes and nothing happened. The table has only 5,000 records, and I can't wait too much time since it block the access to the table.
  I have a test database (equal to production), and it worked very quickly.
  The postgres version is 9.5.6.</p>