Restored (in the modified form) the code wrongly removed three years …

…ago. This acts as a complimentary fix for CORE-4142.

src/jrd/opt.cpp

@@ -661,21 +661,34 @@ RecordSource* OPT_compile(thread_db* tdbb, CompilerScratch* csb, RseNode* rse,
 	for (StreamList::iterator i = opt->subStreams.begin(); i != opt->subStreams.end(); ++i)
 		csb->csb_rpt[*i].activate();
 
+	bool sortCanBeUsed = true;
+	SortNode* const orgSortNode = sort;
+
+	// When DISTINCT and ORDER BY are done on different fields,
+	// and ORDER BY can be mapped to an index, then the records
+	// are returned in the wrong order because DISTINCT sort is
+	// performed after the navigational walk of the index.
+	// For that reason, we need to de-optimize this case so that
+	// ORDER BY does not use an index.
+	if (sort && project)
+	{
+		sort = NULL;
+		sortCanBeUsed = false;
+	}
+
 	// outer joins require some extra processing
 	if (rse->rse_jointype != blr_inner)
 		rsb = gen_outer(tdbb, opt, rse, rivers, &sort);
 	else
 	{
-		bool sort_can_be_used = true;
-		SortNode* const saved_sort_node = sort;
-
 		// AB: If previous rsb's are already on the stack we can't use
 		// a navigational-retrieval for an ORDER BY because the next
 		// streams are JOINed to the previous ones
 		if (rivers.hasData())
 		{
 			sort = NULL;
-			sort_can_be_used = false;
+			sortCanBeUsed = false;
+
 			// AB: We could already have multiple rivers at this
 			// point so try to do some hashing or sort/merging now.
 			while (gen_equi_join(tdbb, opt, rivers))
@@ -706,7 +719,7 @@ RecordSource* OPT_compile(thread_db* tdbb, CompilerScratch* csb, RseNode* rse,
 			if (dependent_streams.getCount() && free_streams.getCount())
 			{
 				sort = NULL;
-				sort_can_be_used = false;
+				sortCanBeUsed = false;
 			}
 
 			if (dependent_streams.getCount())
@@ -751,16 +764,14 @@ RecordSource* OPT_compile(thread_db* tdbb, CompilerScratch* csb, RseNode* rse,
 
 		rsb = CrossJoin(csb, rivers).getRecordSource();
 
-		// Assign the sort node back if it wasn't used by the index navigation
-		if (saved_sort_node && !sort_can_be_used)
-		{
-			sort = saved_sort_node;
-		}
-
 		// Pick up any residual boolean that may have fallen thru the cracks
 		rsb = gen_residual_boolean(tdbb, opt, rsb);
 	}
 
+	// Assign the sort node back if it wasn't used by the index navigation
+	if (orgSortNode && !sortCanBeUsed)
+		sort = orgSortNode;
+
 	// if the aggregate was not optimized via an index, get rid of the
 	// sort and flag the fact to the calling routine
 	if (aggregate && sort)