Aklakan commented on code in PR #2925:
URL: https://github.com/apache/jena/pull/2925#discussion_r1916869473


##########
jena-arq/src/main/java/org/apache/jena/sparql/engine/iterator/QueryIterLateral.java:
##########
@@ -292,17 +295,33 @@ public Op transform(OpTable opTable) {
             // By the assignment restriction, the binding only needs to be 
added to each row of the table.
             Table table = opTable.getTable();
             // Table vars.
-            List<Var> vars = new ArrayList<>(table.getVars());
-            binding.vars().forEachRemaining(vars::add);
-            TableN table2 = new TableN(vars);
+            List<Var> tableVars = table.getVars();
+            List<Var> vars = new ArrayList<>(tableVars);
+
+            // Track variables that appear both in the table and the binding.
+            List<Var> commonVars = new ArrayList<>();
+
+            // Index variables in a set if there are more than a few of them.
+            Collection<Var> tableVarsIndex = tableVars.size() > 4 ? new 
HashSet<>(tableVars) : tableVars;
+            binding.vars().forEachRemaining(v -> {
+                if (tableVarsIndex.contains(v)) {
+                    commonVars.add(v);
+                } else {
+                    vars.add(v);
+                }
+            });
+
+            List<Binding> bindings = new ArrayList<>(table.size());
             BindingBuilder builder = BindingFactory.builder();
             table.iterator(null).forEachRemaining(row->{
-                builder.reset();
-                builder.addAll(row);
-                builder.addAll(binding);
-                table2.addBinding(builder.build());
+                if (Algebra.compatible(row, binding, commonVars.iterator())) {

Review Comment:
   There is potentially another can of worms I was about to open w.r.t. sep7:
   
   Perhaps its worthy of opening an issue and discussion on sparql-dev.  The 
reason I added the compatibility check was because I had the following query in 
mind, which for reasons I do not yet understand is currently prohibited.
   Removing the restrictions in `SyntaxVarScope.checkLATERAL` on `Element_Data` 
(and `Element_Bind`) and adding the check for compatible bindings (effectively 
filtering bindings of tables down to those compatible with the current row; as 
done in the PR) naturally makes the query work.
   
   ```sparql
   SELECT * {
     VALUES ?department {
       <urn:dept1>
       <urn:dept2>
     }
     LATERAL {
       SELECT * {
         VALUES (?department ?employee) {
           ( <urn:dept1> <urn:person1> )
           ( <urn:dept1> <urn:person2> )
           ( <urn:dept2> <urn:person1> )
         }
       } ORDER BY ?employee LIMIT 1
     }
   }
   ```
   
   | department  | employee      |
   |--------------------|---------|
   | <urn:dept1> | <urn:person1> |
   | <urn:dept2> | <urn:person1> |
   
   



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