Fixed #35559 -- Avoided unnecessary query on sliced union of empty queries.

While refs #34125 focused on the SQL correctness of slicing of union of
potentially empty queries it missed an optimization opportunity to avoid
performing a query at all when all queries are empty.

Thanks Lucidiot for the report.

django/db/models/sql/compiler.py

@@ -583,20 +583,49 @@ class SQLCompiler:
                     raise DatabaseError(
                         "ORDER BY not allowed in subqueries of compound statements."
                     )
-        elif self.query.is_sliced and combinator == "union":
-            for compiler in compilers:
-                # A sliced union cannot have its parts elided as some of them
-                # might be sliced as well and in the event where only a single
-                # part produces a non-empty resultset it might be impossible to
-                # generate valid SQL.
-                compiler.elide_empty = False
-        parts = ()
-        selected = self.query.selected
+        parts = []
+        empty_compiler = None
         for compiler in compilers:
             try:
+                parts.append(self._get_combinator_part_sql(compiler))
+            except EmptyResultSet:
+                # Omit the empty queryset with UNION and with DIFFERENCE if the
+                # first queryset is nonempty.
+                if combinator == "union" or (combinator == "difference" and parts):
+                    empty_compiler = compiler
+                    continue
+                raise
+        if not parts:
+            raise EmptyResultSet
+        elif len(parts) == 1 and combinator == "union" and self.query.is_sliced:
+            # A sliced union cannot be composed of a single component because
+            # in the event the later is also sliced it might result in invalid
+            # SQL due to the usage of multiple LIMIT clauses. Prevent that from
+            # happening by always including an empty resultset query to force
+            # the creation of an union.
+            empty_compiler.elide_empty = False
+            parts.append(self._get_combinator_part_sql(empty_compiler))
+        combinator_sql = self.connection.ops.set_operators[combinator]
+        if all and combinator == "union":
+            combinator_sql += " ALL"
+        braces = "{}"
+        if not self.query.subquery and features.supports_slicing_ordering_in_compound:
+            braces = "({})"
+        sql_parts, args_parts = zip(
+            *((braces.format(sql), args) for sql, args in parts)
+        )
+        result = [" {} ".format(combinator_sql).join(sql_parts)]
+        params = []
+        for part in args_parts:
+            params.extend(part)
+        return result, params
+
+    def _get_combinator_part_sql(self, compiler):
+        features = self.connection.features
         # If the columns list is limited, then all combined queries
         # must have the same columns list. Set the selects defined on
         # the query on all combined queries, if not already set.
+        selected = self.query.selected
         if selected is not None and compiler.query.selected is None:
             compiler.query = compiler.query.clone()
             compiler.query.set_values(selected)
@@ -613,34 +642,9 @@ class SQLCompiler:
                 or not features.supports_slicing_ordering_in_compound
             ):
                 part_sql = "({})".format(part_sql)
-                elif (
-                    self.query.subquery
-                    and features.supports_slicing_ordering_in_compound
-                ):
+        elif self.query.subquery and features.supports_slicing_ordering_in_compound:
             part_sql = "({})".format(part_sql)
-                parts += ((part_sql, part_args),)
-            except EmptyResultSet:
-                # Omit the empty queryset with UNION and with DIFFERENCE if the
-                # first queryset is nonempty.
-                if combinator == "union" or (combinator == "difference" and parts):
-                    continue
-                raise
-        if not parts:
-            raise EmptyResultSet
-        combinator_sql = self.connection.ops.set_operators[combinator]
-        if all and combinator == "union":
-            combinator_sql += " ALL"
-        braces = "{}"
-        if not self.query.subquery and features.supports_slicing_ordering_in_compound:
-            braces = "({})"
-        sql_parts, args_parts = zip(
-            *((braces.format(sql), args) for sql, args in parts)
-        )
-        result = [" {} ".format(combinator_sql).join(sql_parts)]
-        params = []
-        for part in args_parts:
-            params.extend(part)
-        return result, params
+        return part_sql, part_args
 
     def get_qualify_sql(self):
         where_parts = []

tests/queries/test_qs_combinators.py

@@ -76,6 +76,12 @@ class QuerySetSetOperationTests(TestCase):
         qs3 = qs1.union(qs2)
         self.assertNumbersEqual(qs3[:1], [0])
 
+    def test_union_all_none_slice(self):
+        qs = Number.objects.filter(id__in=[])
+        with self.assertNumQueries(0):
+            self.assertSequenceEqual(qs.union(qs), [])
+            self.assertSequenceEqual(qs.union(qs)[0:0], [])
+
     def test_union_empty_filter_slice(self):
         qs1 = Number.objects.filter(num__lte=0)
         qs2 = Number.objects.filter(pk__in=[])