Why are these queries so different in time?

Hi, I have a question about performance querying a 7.4 database. The  
orginal generated query was

SELECT DISTINCT _compound0.object_id AS "ObjectId"  FROM    amaze._compound _compound0    LEFT JOIN amaze._product
_product7ON (_compound0.object_id =  
 
_product7.compound)    LEFT JOIN amaze._database_object _database_object11 ON  
(_product7.reaction = _database_object11.object_id)    LEFT JOIN amaze._educt _educt2 ON (_compound0.object_id =  
_educt2.compound)    LEFT JOIN amaze._database_object _database_object6 ON  
(_educt2.reaction = _database_object6.object_id)  WHERE
(    _database_object6.label = '2-Isopropylmalate + CoA <= Acetyl-CoA  
+ 3-Methyl-2-oxobutanoate + H2O'      OR    _database_object11.label = '2-Isopropylmalate + CoA <= Acetyl-CoA  
+ 3-Methyl-2-oxobutanoate + H2O'
);

This on take a huge time to perform, which may come to a timeout on the  
front-end application that uses the database.
So, I decided to modify manually the query like this:

SELECT DISTINCT _compound0.object_id AS "ObjectId"  FROM    amaze._compound _compound0    LEFT JOIN amaze._product
_product7ON (_compound0.object_id =  
 
_product7.compound)    LEFT JOIN amaze._database_object _database_object11 ON  
(_product7.reaction = _database_object11.object_id)  WHERE
(    _database_object11.label = '2-Isopropylmalate + CoA <= Acetyl-CoA  
+ 3-Methyl-2-oxobutanoate + H2O'
)
UNION
SELECT DISTINCT _compound0.object_id AS "ObjectId"  FROM    amaze._compound _compound0    LEFT JOIN amaze._educt
_educt2ON (_compound0.object_id =  
 
_educt2.compound)    LEFT JOIN amaze._database_object _database_object6 ON  
(_educt2.reaction = _database_object6.object_id)  WHERE
(    _database_object6.label = '2-Isopropylmalate + CoA <= Acetyl-CoA  
+ 3-Methyl-2-oxobutanoate + H2O'
)

This should give the same result set, but it's really faster than the  
previous one, more than one thousand time faster.
Is there a reason for this huge difference of performance?

Thanks in advance.

-- 
Olivier Hubaut
North Bears Team
SCMBB - ULB