Fixed #28670 -- Added FETCH/OFFSET support on Oracle.

Thanks Tim Graham for the review.

4 files changed, 13 insertions, 71 deletions

diff --git a/django/db/backends/oracle/compiler.py b/django/db/backends/oracle/compiler.py
deleted file mode 100644
index b568e59e9e..0000000000
--- a/django/db/backends/oracle/compiler.py
+++ /dev/null
@@ -1,62 +0,0 @@
-from django.db import NotSupportedError
-from django.db.models.sql import compiler
-
-
-class SQLCompiler(compiler.SQLCompiler):
-    def as_sql(self, with_limits=True, with_col_aliases=False):
-        """
-        Create the SQL for this query. Return the SQL string and list
-        of parameters.  This is overridden from the original Query class
-        to handle the additional SQL Oracle requires to emulate LIMIT
-        and OFFSET.
-
-        If 'with_limits' is False, any limit/offset information is not
-        included in the query.
-        """
-        # The `do_offset` flag indicates whether we need to construct
-        # the SQL needed to use limit/offset with Oracle.
-        do_offset = with_limits and (self.query.high_mark is not None or self.query.low_mark)
-        if not do_offset:
-            sql, params = super().as_sql(with_limits=False, with_col_aliases=with_col_aliases)
-        elif not self.connection.features.supports_select_for_update_with_limit and self.query.select_for_update:
-            raise NotSupportedError(
-                'LIMIT/OFFSET is not supported with select_for_update on this '
-                'database backend.'
-            )
-        else:
-            sql, params = super().as_sql(with_limits=False, with_col_aliases=True)
-            # Wrap the base query in an outer SELECT * with boundaries on
-            # the "_RN" column.  This is the canonical way to emulate LIMIT
-            # and OFFSET on Oracle.
-            high_where = ''
-            if self.query.high_mark is not None:
-                high_where = 'WHERE ROWNUM <= %d' % (self.query.high_mark,)
-
-            if self.query.low_mark:
-                sql = (
-                    'SELECT * FROM (SELECT "_SUB".*, ROWNUM AS "_RN" FROM (%s) '
-                    '"_SUB" %s) WHERE "_RN" > %d' % (sql, high_where, self.query.low_mark)
-                )
-            else:
-                # Simplify the query to support subqueries if there's no offset.
-                sql = (
-                    'SELECT * FROM (SELECT "_SUB".* FROM (%s) "_SUB" %s)' % (sql, high_where)
-                )
-
-        return sql, params
-
-
-class SQLInsertCompiler(compiler.SQLInsertCompiler, SQLCompiler):
-    pass
-
-
-class SQLDeleteCompiler(compiler.SQLDeleteCompiler, SQLCompiler):
-    pass
-
-
-class SQLUpdateCompiler(compiler.SQLUpdateCompiler, SQLCompiler):
-    pass
-
-
-class SQLAggregateCompiler(compiler.SQLAggregateCompiler, SQLCompiler):
-    pass
diff --git a/django/db/backends/oracle/features.py b/django/db/backends/oracle/features.py
index 71421f0df8..cb2fa7d558 100644
--- a/django/db/backends/oracle/features.py
+++ b/django/db/backends/oracle/features.py
@@ -12,7 +12,6 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     has_select_for_update_of = True
     select_for_update_of_column = True
     can_return_id_from_insert = True
-    allow_sliced_subqueries = False
     can_introspect_autofield = True
     supports_subqueries_in_group_by = False
     supports_transactions = True
diff --git a/django/db/backends/oracle/operations.py b/django/db/backends/oracle/operations.py
index 5c0f6accae..fa29a8925a 100644
--- a/django/db/backends/oracle/operations.py
+++ b/django/db/backends/oracle/operations.py
@@ -13,8 +13,6 @@ from .utils import BulkInsertMapper, InsertIdVar, Oracle_datetime
 
 
 class DatabaseOperations(BaseDatabaseOperations):
-    compiler_module = "django.db.backends.oracle.compiler"
-
     # Oracle uses NUMBER(11) and NUMBER(19) for integer fields.
     integer_field_ranges = {
         'SmallIntegerField': (-99999999999, 99999999999),
@@ -233,8 +231,15 @@ END;
         else:
             return "%s"
 
+    def no_limit_value(self):
+        return None
+
     def limit_offset_sql(self, low_mark, high_mark):
-        return ''
+        fetch, offset = self._get_limit_offset_params(low_mark, high_mark)
+        return '%s%s' % (
+            (' OFFSET %d ROWS' % offset) if offset else '',
+            (' FETCH FIRST %d ROWS ONLY' % fetch) if fetch else '',
+        )
 
     def last_executed_query(self, cursor, sql, params):
         # https://cx-oracle.readthedocs.io/en/latest/cursor.html#Cursor.statement
diff --git a/django/db/models/sql/compiler.py b/django/db/models/sql/compiler.py
index 078a8477af..93ba60638a 100644
--- a/django/db/models/sql/compiler.py
+++ b/django/db/models/sql/compiler.py
@@ -439,6 +439,8 @@ class SQLCompiler:
         try:
             extra_select, order_by, group_by = self.pre_sql_setup()
             for_update_part = None
+            # Is a LIMIT/OFFSET clause needed?
+            with_limit_offset = with_limits and (self.query.high_mark is not None or self.query.low_mark)
             combinator = self.query.combinator
             features = self.connection.features
             if combinator:
@@ -479,7 +481,7 @@ class SQLCompiler:
                     if self.connection.get_autocommit():
                         raise TransactionManagementError('select_for_update cannot be used outside of a transaction.')
 
-                    if with_limits and not self.connection.features.supports_select_for_update_with_limit:
+                    if with_limit_offset and not self.connection.features.supports_select_for_update_with_limit:
                         raise NotSupportedError(
                             'LIMIT/OFFSET is not supported with '
                             'select_for_update on this database backend.'
@@ -531,10 +533,8 @@ class SQLCompiler:
                     params.extend(o_params)
                 result.append('ORDER BY %s' % ', '.join(ordering))
 
-            if with_limits:
-                limit_offset_sql = self.connection.ops.limit_offset_sql(self.query.low_mark, self.query.high_mark)
-                if limit_offset_sql:
-                    result.append(limit_offset_sql)
+            if with_limit_offset:
+                result.append(self.connection.ops.limit_offset_sql(self.query.low_mark, self.query.high_mark))
 
             if for_update_part and not self.connection.features.for_update_after_from:
                 result.append(for_update_part)