[jira] Commented: (OPENJPA-1483) count (Distinct e) in JPQL gives wrong result when the id field is a compound primary key

["Fay Wang (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/OPENJPA-1483?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12805976#action_12805976 ] 

Fay Wang commented on OPENJPA-1483:
-----------------------------------

Limited support for count(distinct compound key) will be supported when count(compound primary key) appears in the projection list. This projection list will have only one projection item, ie, count, in it.  OpenJPA will use generate the following SQL: 

(1) 
String jpql = "SELECT COUNT (DISTINCT e) FROM G2 e";
generated SQL: 
SELECT COUNT(*) FROM G2 t0

(2) 
String jpql = "SELECT COUNT (DISTINCT f1.g2) FROM F1 f1"; 
generated SQL:
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) 

For count(compound key), OpenJPA will generate count(*) in the SQL.

An unsupported exception will be thrown in all other situations.

> count (Distinct e) in JPQL gives wrong result when the id field is a compound primary key
> -----------------------------------------------------------------------------------------
>
>                 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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