Fixed #30157 -- Dropped support for Oracle 12.1.

Thanks Tim Graham for the review.

django/db/backends/oracle/compiler.py

-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 restriction in Oracle 12.1 and emulate LIMIT and OFFSET with
-        a subquery.
-
-        If 'with_limits' is False, any limit/offset information is not included
-        in the query.
-        """
-        # Whether the query must be constructed using limit/offset.
-        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

django/db/backends/oracle/features.py

 from django.db.backends.base.features import BaseDatabaseFeatures
 from django.db.utils import InterfaceError
-from django.utils.functional import cached_property
 
 
 class DatabaseFeatures(BaseDatabaseFeatures):
@@ -56,15 +55,4 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     supports_ignore_conflicts = False
     max_query_params = 2**16 - 1
     supports_partial_indexes = False
-
-    @cached_property
-    def has_fetch_offset_support(self):
-        return self.connection.oracle_version >= (12, 2)
-
-    @cached_property
-    def allow_sliced_subqueries_with_in(self):
-        return self.has_fetch_offset_support
-
-    @cached_property
-    def supports_slicing_ordering_in_compound(self):
-        return self.has_fetch_offset_support
+    supports_slicing_ordering_in_compound = True

django/db/backends/oracle/operations.py

@@ -580,9 +580,3 @@ END;
         if fields:
             return self.connection.features.max_query_params // len(fields)
         return len(objs)
-
-    @cached_property
-    def compiler_module(self):
-        if self.connection.features.has_fetch_offset_support:
-            return super().compiler_module
-        return 'django.db.backends.oracle.compiler'

docs/ref/contrib/gis/install/index.txt

@@ -60,7 +60,7 @@ Database            Library Requirements            Supported Versions  Notes
 ==================  ==============================  ==================  =========================================
 PostgreSQL          GEOS, GDAL, PROJ.4, PostGIS     9.5+                Requires PostGIS.
 MySQL               GEOS, GDAL                      5.6+                Not OGC-compliant; :ref:`limited functionality <mysql-spatial-limitations>`.
-Oracle              GEOS, GDAL                      12.1+               XE not supported.
+Oracle              GEOS, GDAL                      12.2+               XE not supported.
 SQLite              GEOS, GDAL, PROJ.4, SpatiaLite  3.8.3+              Requires SpatiaLite 4.3+
 ==================  ==============================  ==================  =========================================
 

docs/ref/databases.txt

@@ -730,7 +730,7 @@ iterator. Your code must handle this.
 Oracle notes
 ============
 
-Django supports `Oracle Database Server`_ versions 12.1 and higher. Version
+Django supports `Oracle Database Server`_ versions 12.2 and higher. Version
 6.0 or higher of the `cx_Oracle`_ Python driver is required.
 
 .. _`Oracle Database Server`: https://www.oracle.com/

docs/releases/3.0.txt

@@ -235,6 +235,12 @@ Dropped support for PostgreSQL 9.4
 Upstream support for PostgreSQL 9.4 ends in December 2019. Django 3.0 supports
 PostgreSQL 9.5 and higher.
 
+Dropped support for Oracle 12.1
+-------------------------------
+
+Upstream support for Oracle 12.1 ends in July 2021. Django 2.2 will be
+supported until April 2022. Django 3.0 officially supports Oracle 12.2 and 18c.
+
 Removed private Python 2 compatibility APIs
 -------------------------------------------