public function walkSelectStatement(SelectStatement $AST)
    {
        $rootComponents = [];
        foreach ($this->_getQueryComponents() as $dqlAlias => $qComp) {
            if (array_key_exists('parent', $qComp) && $qComp['parent'] === null && $qComp['nestingLevel'] == 0) {
                $rootComponents[] = [$dqlAlias => $qComp];
            }
        }

        if (count($rootComponents) > 1) {
            throw new \RuntimeException('Cannot count query which selects two FROM components, cannot make distinction');
        }

        $root = reset($rootComponents);
        $parentName = key($root);
        $parent = current($root);

        $pathExpression = new PathExpression(
            PathExpression::TYPE_STATE_FIELD | PathExpression::TYPE_SINGLE_VALUED_ASSOCIATION, $parentName,
            $parent['metadata']->getSingleIdentifierFieldName()
        );
        $pathExpression->type = PathExpression::TYPE_STATE_FIELD;

        // Remove the variables which are not used by other clauses
        foreach ($AST->selectClause->selectExpressions as $key => $selectExpression) {
            if ($selectExpression->fieldIdentificationVariable === null) {
                unset($AST->selectClause->selectExpressions[$key]);
            } elseif ($selectExpression->expression instanceof PathExpression) {
                $groupByClause[] = $selectExpression->expression;
            }
        }

        // Put the count expression in the first position
        $distinct = $this->_getQuery()->getHint(self::HINT_DISTINCT);
        array_unshift($AST->selectClause->selectExpressions,
            new SelectExpression(
                new AggregateExpression('count', $pathExpression, $distinct), null
            )
        );

        $groupByClause[] = $pathExpression;
        $AST->groupByClause = new \Doctrine\ORM\Query\AST\GroupByClause($groupByClause);

        // ORDER BY is not needed, only increases query execution through unnecessary sorting.
        $AST->orderByClause = null;
    }