correctly produce alter table sql to update column comment under postgres

lib/Doctrine/DBAL/Platforms/PostgreSqlPlatform.php

@@ -591,11 +591,17 @@ class PostgreSqlPlatform extends AbstractPlatform
                 }
             }
 
-            if ($columnDiff->hasChanged('comment')) {
+            $oldComment = null;
+            $newComment = $this->getColumnComment($column);
+            if (null !== $columnDiff->fromColumn) {
+                $oldComment = $this->getColumnComment($columnDiff->fromColumn);
+            }
+
+            if ($columnDiff->hasChanged('comment') || ($columnDiff->fromColumn !== null && $oldComment !== $newComment)) {
                 $commentsSQL[] = $this->getCommentOnColumnSQL(
                     $diff->getName($this)->getQuotedName($this),
                     $column->getQuotedName($this),
-                    $this->getColumnComment($column)
+                    $newComment
                 );
             }
 

tests/Doctrine/Tests/DBAL/Platforms/AbstractPostgreSqlPlatformTestCase.php

@@ -853,6 +853,29 @@ abstract class AbstractPostgreSqlPlatformTestCase extends AbstractPlatformTestCa
         );
     }
 
+    /**
+     * @group 3158
+     */
+    public function testAltersTableColumnCommentIfRequiredByType()
+    {
+        $table1 = new Table('"foo"', [new Column('"bar"', Type::getType('datetime'))]);
+        $table2 = new Table('"foo"', [new Column('"bar"', Type::getType('datetime_immutable'))]);
+
+        $comparator = new Comparator();
+
+        $tableDiff = $comparator->diffTable($table1, $table2);
+
+        $this->assertInstanceOf('Doctrine\DBAL\Schema\TableDiff', $tableDiff);
+        $this->assertSame(
+            [
+                'ALTER TABLE "foo" ALTER "bar" TYPE TIMESTAMP(0) WITHOUT TIME ZONE',
+                'ALTER TABLE "foo" ALTER "bar" DROP DEFAULT',
+                'COMMENT ON COLUMN "foo"."bar" IS \'(DC2Type:datetime_immutable)\'',
+            ],
+            $this->_platform->getAlterTableSQL($tableDiff)
+        );
+    }
+
     /**
      * {@inheritdoc}
      */