Fetch field contents without loading whole entity.

field-contents.php

<?php
/**
 * Get the content of a field for a given array of entity ids. If the field
 * contains multiple values for a given entity id, then only the latest is
 * preserved.
 *
 * @param string $field_name
 *   The name of the field you need to retrieve.
 * @param array $entity_ids
 *   An array containing the entity ids to look for.
 * @param string $column_name
 *   Name of the column in the database that holds the value for the field.
 *   Defaults to 'value'.
 *
 * @return array
 *   A keyed array with keys representing the entity ids and the values
 *   representing the value of the field.
 */
function entity_field_single_value($field_name, $entity_ids, $column_name = 'value') {
  /** @var string $field_tablename Name of the SQL table that holds the field information. This will probably become 'field_data_field_whatever'. */
  $field_tablename = _field_sql_storage_tablename(field_info_field($field_name));
  /** @var string $field_columnname Name of the SQL column that holds the field information. This will probably become 'field_whatever_value'. */
  $field_columnname = _field_sql_storage_columnname($field_name, $column_name);
  return db_select($field_tablename, 'f')
    ->fields('f', array('entity_id', $field_columnname))
    ->condition('f.entity_id', $entity_ids, 'IN')
    ->execute()
    ->fetchAllKeyed();
}