JPQL "DISTINCT" returns only one result - jpa

Question

I am confused by DISTINCT in JPQL. I have two JPQL queries identical except for "DISTINCT" in one of them:
String getObjectsForFlow =
"SELECT " +
" se.componentID " +
"FROM " +
" StatisticsEvent se " +
"WHERE " +
" se.serverID IS NOT NULL " +
" AND se.flowID = :uuid " +
" AND se.componentID IS NOT NULL " +
"ORDER BY " +
" se.timeStamp desc ";
String getObjectsForFlowDistinct =
"SELECT DISTINCT " +
" se.componentID " +
"FROM " +
" StatisticsEvent se " +
"WHERE " +
" se.serverID IS NOT NULL " +
" AND se.flowID = :uuid " +
" AND se.componentID IS NOT NULL " +
"ORDER BY " +
" se.timeStamp desc ";
I run a little code to get the results from each query and dump them to stdout, and I get many rows with some duplicates for non-distinct, but for distinct I get only one row which is part of the non-distinct list.
NOT DISTINCT
::: 01e2e915-35c1-6cf0-9d0e-14109fdb7235
::: 01e2e915-35c1-6cf0-9d0e-14109fdb7235
::: 01e2e915-35d9-afe0-9d0e-14109fdb7235
::: 01e2e915-35d9-afe0-9d0e-14109fdb7235
::: 01e2e915-35bd-c370-9d0e-14109fdb7235
::: 01e2e915-35bd-c370-9d0e-14109fdb7235
::: 01e2e915-35aa-1460-9d0e-14109fdb7235
::: 01e2e915-35d1-2460-9d0e-14109fdb7235
::: 01e2e915-35e1-7810-9d0e-14109fdb7235
::: 01e2e915-35e1-7810-9d0e-14109fdb7235
::: 01e2e915-35d0-12f0-9d0e-14109fdb7235
::: 01e2e915-35b0-cb20-9d0e-14109fdb7235
::: 01e2e915-35a8-66b0-9d0e-14109fdb7235
::: 01e2e915-35a8-66b0-9d0e-14109fdb7235
::: 01e2e915-35e2-6270-9d0e-14109fdb7235
::: 01e2e915-357f-33d0-9d0e-14109fdb7235
DISTINCT
::: 01e2e915-35e2-6270-9d0e-14109fdb7235
Where are the other entries? I would expect a DISTINCT list containing eleven (I think) entries.

Answer 1

Double check equals() method on your StatisticsEvent entity class. Maybe those semantically different values returns same when equals() is called hence producing this behavior

Answer 2

The problem was the "ORDER BY se.timeStamp" clause. To fulfill the request, JPQL added the ORDER BY field to the SELECT DISTINCT clause.
This is like a border case in the interplay between JPQL and SQL. The JPQL syntax clearly applies the DISTINCT modifier only to se.componentID, but when translated into SQL the ORDER BY field gets inserted.
I am surprised that the ORDER BY field had to be selected at all. Some databases can return a data set ORDERed by a field not in the SELECTion. Oracle can do so. My underlying database is Derby -- could this be a limitation in Derby?

Answer 3

Oracle does not support SELECT DISTINCT with an order by unless the order by columns are in the SELECT. Not sure if any databases do. It will work in Oracle if the DISTINCT is not required (does not run because rows are unique), but if it needs to run you will get an error.
You will get, "ORA-01791: not a SELECTed expression"
If you are using EclipseLink this functionality is controlled by the DatabasPlatform method,
shouldSelectDistinctIncludeOrderBy()
You can extend your platform to return false if your database does not require this.
Still, I don't see how adding the TIMESTAMP will change the query results?

Answer 4

Both queries are incorrect JPQL queries, because ORDER BY clause refers to the item that is not on select list. JPA 2.0 specification contains example that matches to this case:
The following two queries are not legal because the orderby_item is
not reflected in the SELECT clause of the query.
SELECT p.product_name
FROM Order o JOIN o.lineItems l JOIN l.product p JOIN o.customer c
WHERE c.lastname = ‘Smith’ AND c.firstname = ‘John’
ORDER BY p.price
SELECT p.product_name
FROM Order o, IN(o.lineItems) l JOIN o.customer c
WHERE c.lastname = ‘Smith’ AND c.firstname = ‘John’
ORDER BY
o.quantity
Of course it would be nicer if if implementation could give clear error message instead of trying to guess what is expected result of incorrect query.