implement batched INSERT..VALUES () () for executemany

the feature is enabled for all built in backends
when RETURNING is used,
except for Oracle that doesn't need it,  and on
psycopg2 and mssql+pyodbc it is used for all INSERT statements,
not just those that use RETURNING.

third party dialects would need to opt in to the new feature
by setting use_insertmanyvalues to True.

Also adds dialect-level guards against using returning
with executemany where we dont have an implementation to
suit it.   execute single w/ returning still defers to the
server without us checking.

Fixes: #6047
Fixes: #7907
Change-Id: I3936d3c00003f02e322f2e43fb949d0e6e568304

1 files changed, 2 insertions, 19 deletions

diff --git a/lib/sqlalchemy/dialects/postgresql/base.py b/lib/sqlalchemy/dialects/postgresql/base.py
index 18a7c0a86..3e43d601f 100644
--- a/lib/sqlalchemy/dialects/postgresql/base.py
+++ b/lib/sqlalchemy/dialects/postgresql/base.py
@@ -1330,7 +1330,6 @@ from typing import List
 from typing import Optional
 
 from . import array as _array
-from . import dml
 from . import hstore as _hstore
 from . import json as _json
 from . import pg_catalog
@@ -1850,24 +1849,6 @@ class PGCompiler(compiler.SQLCompiler):
 
         return target_text
 
-    @util.memoized_property
-    def _is_safe_for_fast_insert_values_helper(self):
-        # don't allow fast executemany if _post_values_clause is
-        # present and is not an OnConflictDoNothing. what this means
-        # concretely is that the
-        # "fast insert executemany helper" won't be used, in other
-        # words we won't convert "executemany()" of many parameter
-        # sets into a single INSERT with many elements in VALUES.
-        # We can't apply that optimization safely if for example the
-        # statement includes a clause like "ON CONFLICT DO UPDATE"
-
-        return self.insert_single_values_expr is not None and (
-            self.statement._post_values_clause is None
-            or isinstance(
-                self.statement._post_values_clause, dml.OnConflictDoNothing
-            )
-        )
-
     def visit_on_conflict_do_nothing(self, on_conflict, **kw):
 
         target_text = self._on_conflict_target(on_conflict, **kw)
@@ -2804,6 +2785,7 @@ class PGDialect(default.DefaultDialect):
     sequences_optional = True
     preexecute_autoincrement_sequences = True
     postfetch_lastrowid = False
+    use_insertmanyvalues = True
 
     supports_comments = True
     supports_constraint_comments = True
@@ -2813,6 +2795,7 @@ class PGDialect(default.DefaultDialect):
 
     supports_empty_insert = False
     supports_multivalues_insert = True
+
     supports_identity_columns = True
 
     default_paramstyle = "pyformat"