render WITH clause after INSERT for INSERT..SELECT on Oracle, MySQL

Fixed INSERT FROM SELECT with CTEs for the Oracle and MySQL dialects, where
the CTE was being placed above the entire statement as is typical with
other databases, however Oracle and MariaDB 10.2 wants the CTE underneath
the "INSERT" segment. Note that the Oracle and MySQL dialects don't yet
work when a CTE is applied to a subquery inside of an UPDATE or DELETE
statement, as the CTE is still applied to the top rather than inside the
subquery.

Also adds test suite support CTEs against backends.

Change-Id: I8ac337104d5c546dd4f0cd305632ffb56ac8bf90
Fixes: #4275
Fixes: #4230

1 files changed, 7 insertions, 2 deletions

diff --git a/lib/sqlalchemy/sql/compiler.py b/lib/sqlalchemy/sql/compiler.py
index a442c65fd..0b98dc51c 100644
--- a/lib/sqlalchemy/sql/compiler.py
+++ b/lib/sqlalchemy/sql/compiler.py
@@ -2105,7 +2105,12 @@ class SQLCompiler(Compiled):
             returning_clause = None
 
         if insert_stmt.select is not None:
-            text += " %s" % self.process(self._insert_from_select, **kw)
+            select_text = self.process(self._insert_from_select, **kw)
+
+            if self.ctes and toplevel and self.dialect.cte_follows_insert:
+                text += " %s%s" % (self._render_cte_clause(), select_text)
+            else:
+                text += " %s" % select_text
         elif not crud_params and supports_default_values:
             text += " DEFAULT VALUES"
         elif insert_stmt._has_multi_parameters:
@@ -2130,7 +2135,7 @@ class SQLCompiler(Compiled):
         if returning_clause and not self.returning_precedes_values:
             text += " " + returning_clause
 
-        if self.ctes and toplevel:
+        if self.ctes and toplevel and not self.dialect.cte_follows_insert:
             text = self._render_cte_clause() + text
 
         self.stack.pop(-1)