title: Liquibase: added autoIncrement column into a table with data already present
body: <p>Hi guys I have some doubt about Liquibase</p>
  <p>I created a table (<em>PostgreSQL</em>) with a classical changeset</p>
  <pre><code>&lt;changeSet id=&quot;create_table&quot;&gt;
      &lt;createTable tableName=&quot;table&quot; schemaName=&quot;schema&quot;&gt;
          &lt;column name=&quot;name&quot; type=&quot;varchar&quot;&gt;
              &lt;constraints nullable=&quot;false&quot;/&gt;
          &lt;/column&gt;
          &lt;column name=&quot;surname&quot; type=&quot;varchar&quot;&gt;
              &lt;constraints nullable=&quot;false&quot;/&gt;
          &lt;/column&gt;
      &lt;/createTable&gt;
  &lt;/changeSet&gt;
  </code></pre>
  <p><strong>I've already added some data to this table</strong>.
  I want to add a new column '<em><strong>id</strong></em>' and make it as a primary key as an autoincrement column.
  How can I do that?</p>
  <p>Because if I try to add a changeset like this:</p>
  <pre><code>&lt;changeSet id=&quot;added_pk&quot;&gt;
      &lt;addColumn tableName=&quot;table&quot;
                 schemaName=&quot;schema&quot;&gt;
          &lt;column name=&quot;id&quot; type=&quot;bigint&quot;&gt;
              &lt;constraints unique=&quot;true&quot; nullable=&quot;false&quot; uniqueConstraintName=&quot;PK_TABLE_ID&quot;/&gt;
          &lt;/column&gt;
      &lt;/addColumn&gt;
      &lt;addAutoIncrement columnDataType=&quot;bigint&quot;
                        columnName=&quot;id&quot;
                        tableName=&quot;table&quot;
                        schemaName=&quot;schema&quot;
                        incrementBy=&quot;1&quot; startWith=&quot;1&quot;/&gt;
  &lt;/changeSet&gt;
  </code></pre>
  <p>I obtain error when I run the script: &quot;<em>column &quot;id&quot; of relation &quot;table&quot; contains null values</em>&quot;.
  It depends of the already existing values. How could I manage this situation? (I prefer to avoid to truncate all the data before add the new column)</p>
  <p>Thanks a lot!
  Regards</p>