[jira] Created: (OPENJPA-1483) count (Distinct e) in JPQL will give wrong result is the id fieldis a compound primary key

[Fay Wang (JIRA)]

count (Distinct e) in JPQL will give wrong result is the id fieldis a compound 
primary key
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                 Key: OPENJPA-1483
                 URL: https://issues.apache.org/jira/browse/OPENJPA-1483
             Project: OpenJPA
          Issue Type: Bug
    Affects Versions: 2.1.0
            Reporter: Fay Wang
             Fix For: 2.1.0


This is a fundamental problem with count when compound primary key is involved. 

        (1) If no relation navigation is involved:
        String jpql = "SELECT COUNT (DISTINCT e) FROM G2 e";

        With the property below:
        <property name="openjpa.jdbc.DBDictionary"
                value="db2(useWildCardForCount=true)" />

        Openjpa will generate the following sql and return the correct count:

        SELECT COUNT(*) FROM G2 t0  optimize for 1 row

        (2) If there is relation navigation invloved:
        String jpql = "SELECT COUNT (DISTINCT f1.g2) FROM F1 f1";
        
        The property of useWildCardForCount will not generate correct sql with 
right result. However, given the object-relational impedance mismatch, there is 
no corresponding SQL construct for count of multiple primary keys, and there is 
no clean and generic solution to solve this problem. The only workaround is to 
use native SQL with table expression:

        SELECT COUNT(*)
        FROM (SELECT DISTINCT G1.G1PK, G1.G2PK  FROM F1 t0 INNER JOIN G2 t1 ON 
t0.G1PK = t1.G1PK AND t0.G2PK = t1.G2PK)) TX;    

        Rather than giving a wrong answer, OpenJPA should give an Unsupported 
exception.



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