DROP TABLE IF EXISTS t2;
CREATE TEMP TABLE t2 (id INT,x xml);

INSERT INTO t2 VALUES (0,
'<tbl>
<comments photo_id="123">
     <comment>this is the first 123 comment</comment>
     <description>this is description 123</description>     
</comments>
<comments photo_id="124">
     <comment>this is the first 124 comment</comment>
     <description>this is description 124</description>
</comments>
</tbl>'::xml);    

INSERT INTO t2 VALUES (1,
'<tbl>
<comments photo_id="123">
     <comment>this is the first 123 comment</comment>
     <description>this is description 123</description>     
</comments>
<comments photo_id="124">
     <comment>this is the first 124 comment</comment>
     <description/>
</comments>
</tbl>'::xml); 

SELECT (xpath('./@photo_id', c.node))[1] AS photo_id
     , unnest(xpath('./comment/text()', c.node)) AS comment
     , unnest(xpath('./description/text()', c.node)) AS description     
FROM  (             
    SELECT unnest(xpath('./comments', x)) AS node
    FROM   t2 where id=0
    ) c;