diff --git c/core/lib/Drupal/Core/Database/StatementPrefetch.php w/core/lib/Drupal/Core/Database/StatementPrefetch.php
index e3d9ded..6433906 100644
--- c/core/lib/Drupal/Core/Database/StatementPrefetch.php
+++ w/core/lib/Drupal/Core/Database/StatementPrefetch.php
@@ -2,7 +2,7 @@
 
 /**
  * @file
- * Definition of Drupal\Core\Database\StatementPrefetch
+ * Definition of Drupal\Core\Database\StatementPrefetch.
  */
 
 namespace Drupal\Core\Database;
@@ -91,6 +91,7 @@ class StatementPrefetch implements \Iterator, StatementInterface {
 
   /**
    * Holds the current fetch style (which will be used by the next fetch).
+   *
    * @see \PDOStatement::fetch()
    *
    * @var int
@@ -98,7 +99,9 @@ class StatementPrefetch implements \Iterator, StatementInterface {
   protected $fetchStyle = \PDO::FETCH_OBJ;
 
   /**
-   * Holds supplementary current fetch options (which will be used by the next fetch).
+   * Holds supplementary current fetch options.
+   *
+   * Fetch options stored will be used by the next fetch.
    *
    * @var Array
    */
@@ -135,6 +138,18 @@ class StatementPrefetch implements \Iterator, StatementInterface {
    */
   public $allowRowCount = FALSE;
 
+  /**
+   * Constructs a StatementPrefetch object.
+   *
+   * @param \PDO $dbh
+   *   PDO database handler.
+   * @param \Drupal\Core\Database\Connection $connection
+   *   Drupal database connection object.
+   * @param string $query
+   *   Database query string.
+   * @param array $driver_options
+   *   Options for database driver (optional).
+   */
   public function __construct(\PDO $dbh, Connection $connection, $query, array $driver_options = array()) {
     $this->dbh = $dbh;
     $this->connection = $connection;
@@ -145,11 +160,13 @@ public function __construct(\PDO $dbh, Connection $connection, $query, array $dr
   /**
    * Executes a prepared statement.
    *
-   * @param $args
-   *   An array of values with as many elements as there are bound parameters in the SQL statement being executed.
-   * @param $options
+   * @param array $args
+   *   An array of values with as many elements as there are bound parameters
+   *   in the SQL statement being executed.
+   * @param array $options
    *   An array of options for this query.
-   * @return
+   *
+   * @return bool
    *   TRUE on success, or FALSE on failure.
    */
   public function execute($args = array(), $options = array()) {
@@ -228,10 +245,11 @@ protected function throwPDOException() {
    * Some drivers (including SQLite) will need to perform some preparation
    * themselves to get the statement right.
    *
-   * @param $query
+   * @param string $query
    *   The query.
    * @param array $args
    *   An array of arguments.
+   *
    * @return \PDOStatement
    *   A PDOStatement object.
    */
@@ -247,6 +265,8 @@ public function getQueryString() {
   }
 
   /**
+   * Sets the fetch mode of the prefetch statement.
+   *
    * @see \PDOStatement::setFetchMode()
    */
   public function setFetchMode($fetchStyle, $a2 = NULL, $a3 = NULL) {
@@ -258,9 +278,11 @@ public function setFetchMode($fetchStyle, $a2 = NULL, $a3 = NULL) {
           $this->defaultFetchOptions['constructor_args'] = $a3;
         }
         break;
+
       case \PDO::FETCH_COLUMN:
         $this->defaultFetchOptions['column'] = $a2;
         break;
+
       case \PDO::FETCH_INTO:
         $this->defaultFetchOptions['object'] = $a2;
         break;
@@ -278,25 +300,29 @@ public function setFetchMode($fetchStyle, $a2 = NULL, $a3 = NULL) {
    * array position in $this->data and format it according to $this->fetchStyle
    * and $this->fetchMode.
    *
-   * @return
-   *  The current row formatted as requested.
+   * @return mixed
+   *   The current row formatted as requested.
    */
   public function current() {
     if (isset($this->currentRow)) {
       switch ($this->fetchStyle) {
         case \PDO::FETCH_ASSOC:
           return $this->currentRow;
+
         case \PDO::FETCH_BOTH:
           // \PDO::FETCH_BOTH returns an array indexed by both the column name
           // and the column number.
           return $this->currentRow + array_values($this->currentRow);
+
         case \PDO::FETCH_NUM:
           return array_values($this->currentRow);
+
         case \PDO::FETCH_LAZY:
           // We do not do lazy as everything is fetched already. Fallback to
           // \PDO::FETCH_OBJ.
         case \PDO::FETCH_OBJ:
           return (object) $this->currentRow;
+
         case \PDO::FETCH_CLASS | \PDO::FETCH_CLASSTYPE:
           $class_name = array_unshift($this->currentRow);
           // Deliberate no break.
@@ -315,11 +341,13 @@ public function current() {
             $result->$k = $v;
           }
           return $result;
+
         case \PDO::FETCH_INTO:
           foreach ($this->currentRow as $k => $v) {
             $this->fetchOptions['object']->$k = $v;
           }
           return $this->fetchOptions['object'];
+
         case \PDO::FETCH_COLUMN:
           if (isset($this->columnNames[$this->fetchOptions['column']])) {
             return $this->currentRow[$k][$this->columnNames[$this->fetchOptions['column']]];
@@ -331,16 +359,25 @@ public function current() {
     }
   }
 
-  /* Implementations of Iterator. */
-
+  /**
+   * Implementations of Iterator.
+   */
   public function key() {
     return $this->currentKey;
   }
 
+  /**
+   * Rewinds the DatabaseStatement.
+   *
+   * This function is a noop. DatabaseStatement cannot be rewound.
+   */
   public function rewind() {
     // Nothing to do: our DatabaseStatement can't be rewound.
   }
 
+  /**
+   * Gets the next row.
+   */
   public function next() {
     if (!empty($this->data)) {
       $this->currentRow = reset($this->data);
@@ -352,6 +389,9 @@ public function next() {
     }
   }
 
+  /**
+   * Checks if current row exists.
+   */
   public function valid() {
     return isset($this->currentRow);
   }
@@ -369,6 +409,9 @@ public function rowCount() {
     }
   }
 
+  /**
+   * {@inheritdoc}
+   */
   public function fetch($fetch_style = NULL, $cursor_orientation = \PDO::FETCH_ORI_NEXT, $cursor_offset = NULL) {
     if (isset($this->currentRow)) {
       // Set the fetch parameter.
@@ -390,6 +433,15 @@ public function fetch($fetch_style = NULL, $cursor_orientation = \PDO::FETCH_ORI
     }
   }
 
+  /**
+   * Returns an entire single column of a result set as an indexed array.
+   *
+   * @param int $index
+   *   The index of the column number to fetch.
+   *
+   * @return mixed
+   *   An indexed array, or FALSE if there is no result set.
+   */
   public function fetchColumn($index = 0) {
     if (isset($this->currentRow) && isset($this->columnNames[$index])) {
       // We grab the value directly from $this->data, and format it.
@@ -402,10 +454,27 @@ public function fetchColumn($index = 0) {
     }
   }
 
+  /**
+   * {@inheritdoc}
+   */
   public function fetchField($index = 0) {
     return $this->fetchColumn($index);
   }
 
+  /**
+   * Fetches the next row and returns it as an object.
+   *
+   * The object will be of the class specified by
+   * StatementInterface::setFetchMode() or stdClass if not specified.
+   *
+   * @param string $class_name
+   *   Name of the class (optional).
+   * @param array $constructor_args
+   *   Constructor arguments (optional).
+   *
+   * @return mixed
+   *   result as an object or FALSE.
+   */
   public function fetchObject($class_name = NULL, $constructor_args = array()) {
     if (isset($this->currentRow)) {
       if (!isset($class_name)) {
@@ -431,6 +500,9 @@ public function fetchObject($class_name = NULL, $constructor_args = array()) {
     }
   }
 
+  /**
+   * {@inheritdoc}
+   */
   public function fetchAssoc() {
     if (isset($this->currentRow)) {
       $result = $this->currentRow;
@@ -442,6 +514,19 @@ public function fetchAssoc() {
     }
   }
 
+  /**
+   * Returns an array containing all of the result set rows.
+   *
+   * @param mixed $fetch_style
+   *   One of the PDO::FETCH_* constants.
+   * @param int $fetch_column
+   *   If $mode is PDO::FETCH_COLUMN, the index of the column to fetch.
+   * @param mixed $constructor_args
+   *   If $mode is PDO::FETCH_CLASS, the arguments to pass to the constructor.
+   *
+   * @return array
+   *   An array of results.
+   */
   public function fetchAll($fetch_style = NULL, $fetch_column = NULL, $constructor_args = NULL) {
     $this->fetchStyle = isset($fetch_style) ? $fetch_style : $this->defaultFetchStyle;
     $this->fetchOptions = $this->defaultFetchOptions;
@@ -466,6 +551,9 @@ public function fetchAll($fetch_style = NULL, $fetch_column = NULL, $constructor
     return $result;
   }
 
+  /**
+   * {@inheritdoc}
+   */
   public function fetchCol($index = 0) {
     if (isset($this->columnNames[$index])) {
       $result = array();
@@ -481,6 +569,9 @@ public function fetchCol($index = 0) {
     }
   }
 
+  /**
+   * {@inheritdoc}
+   */
   public function fetchAllKeyed($key_index = 0, $value_index = 1) {
     if (!isset($this->columnNames[$key_index]) || !isset($this->columnNames[$value_index]))
       return array();
@@ -497,6 +588,9 @@ public function fetchAllKeyed($key_index = 0, $value_index = 1) {
     return $result;
   }
 
+  /**
+   * {@inheritdoc}
+   */
   public function fetchAllAssoc($key, $fetch_style = NULL) {
     $this->fetchStyle = isset($fetch_style) ? $fetch_style : $this->defaultFetchStyle;
     $this->fetchOptions = $this->defaultFetchOptions;