title: Attempted conditional constraint is not an indexable operation issue in dynamodb
body: <p>In DynamoDB my query is not working with ComparisonOperator other than <code>EQ</code> in PHP. I am using Global Secondary Indexes:</p>
  
  <pre><code>$response = $client-&gt;query(array(
      "TableName" =&gt; $tableName,
      "IndexName" =&gt; "topRating", // GSI index name
      "KeyConditions" =&gt; array(
          "rating" =&gt; array(
              "ComparisonOperator" =&gt; "NE", // Only 'EQ' works
              "AttributeValueList" =&gt; array(
                  array(Type::STRING =&gt; "1186") // Sample value
              )
          )
      ),
      "ScanIndexForward" =&gt; true,
      "limit" =&gt; 5
  ));
  </code></pre>
  
  <p>When creating the table I used <code>rating</code> as hash key for creating <code>topRating</code> GSI</p>
  
  <p>The response is as follows:</p>
  
  <blockquote>
    <p>Fatal error: Uncaught Aws\DynamoDb\Exception\ValidationException: AWS 
    Error Code: ValidationException, Status Code: 400, AWS Request ID: 
    XXXXXYYYYYZZZZ, AWS Error Type:  client, AWS Error Message: Attempted
    conditional constraint is not an  indexable operation, User-Agent:
    aws-sdk-php2/2.7.0 Guzzle/3.9.2  curl/7.19.7 PHP/5.4.30 thrown in ....</p>
  </blockquote>