[I] `DISTINCT` with `ORDER BY` on a non-injective expression is rewritten to `REDUCED` and leaks duplicate rows [jena]

["ting668 (via GitHub)" <gi...@apache.org>]

ting668 opened a new issue, #4242:
URL: https://github.com/apache/jena/issues/4242

   ### Version
   
   6.2.0
   
   ### What happened?
   
   **Reporter:** ting668
   
   ## Environment
   
   - Apache Jena Version: `6.2.0`
   - Java Version: `21.0.12+8-LTS`
   
   ## Description
   
   While testing Apache Jena using a method based on attribute-constraint analysis, I found that `SELECT DISTINCT` combined with `ORDER BY` on a non-injective expression leaks duplicate rows: with `ORDER BY STRLEN(?v) ?tag`, the value `"a"^^xsd:string` is returned twice.
   
   `DISTINCT` must eliminate duplicates among the projected solutions, so each distinct value of `?v` must appear exactly once, independent of the ordering.
   
   ## How to Reproduce and Expected Behavior
   
   **Note:** The queries below are a **minimized, simplified example** reproducing the bug.
   
   Buggy query (with the default graph loaded with the N-Triples data below):
   
   ```sparql
   SELECT DISTINCT ?v WHERE { ?s <urn:p> ?v . ?s <urn:q> ?tag . } ORDER BY STRLEN(?v) ?tag
   ```
   
   Data (default graph, N-Triples):
   
   ```nt
   <urn:s0> <urn:p> "a"^^<http://www.w3.org/2001/XMLSchema#string> .
   <urn:s1> <urn:p> "b"^^<http://www.w3.org/2001/XMLSchema#string> .
   <urn:s2> <urn:p> "a"^^<http://www.w3.org/2001/XMLSchema#string> .
   <urn:s0> <urn:q> "1"^^<http://www.w3.org/2001/XMLSchema#integer> .
   <urn:s1> <urn:q> "1"^^<http://www.w3.org/2001/XMLSchema#integer> .
   <urn:s2> <urn:q> "2"^^<http://www.w3.org/2001/XMLSchema#integer> .
   ```
   
   Expected behavior: the two `"a"^^xsd:string` solutions collapse under `DISTINCT`, leaving two rows:
   
   ```text
   [{"v":"\"a\"^^xsd:string"},{"v":"\"b\"^^xsd:string"}]
   ```
   
   Actual behavior: the duplicate `"a"^^xsd:string` row is returned:
   
   ```text
   [{"v":"\"a\"^^xsd:string"},{"v":"\"a\"^^xsd:string"},{"v":"\"b\"^^xsd:string"}]
   ```
   
   This suggests that the `DISTINCT` is compiled to `REDUCED` even though the `ORDER BY` expression `STRLEN(?v)` is not injective on the projected values: `REDUCED` does not guarantee duplicate elimination, so the duplicate `"a"` row leaks into the result.
   
   
   ### Relevant output and stacktrace
   
   ```shell
   
   ```
   
   ### Are you interested in making a pull request?
   
   None


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Re: [I] `DISTINCT` with `ORDER BY` on a non-injective expression is rewritten to `REDUCED` and leaks duplicate rows [jena]

["afs (via GitHub)" <gi...@apache.org>]

afs commented on issue #4242:
URL: https://github.com/apache/jena/issues/4242#issuecomment-5752138322

   Confirmed


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