Deprecate ``DISTINCT ON`` when not targeting PostgreSQL

Deprecate usage of ``DISTINCT ON`` in dialect other than PostgreSQL.
Previously this was silently ignored.
Deprecate old usage of string distinct in MySQL dialect

Fixes: #4002
Change-Id: I38fc64aef75e77748083c11d388ec831f161c9c9

2 files changed, 13 insertions, 1 deletions

diff --git a/lib/sqlalchemy/sql/compiler.py b/lib/sqlalchemy/sql/compiler.py
index bc16b1429..23eff773c 100644
--- a/lib/sqlalchemy/sql/compiler.py
+++ b/lib/sqlalchemy/sql/compiler.py
@@ -2868,7 +2868,16 @@ class SQLCompiler(Compiled):
         before column list.
 
         """
-        return select._distinct and "DISTINCT " or ""
+        if select._distinct_on:
+            util.warn_deprecated(
+                "DISTINCT ON is currently supported only by the PostgreSQL "
+                "dialect.  Use of DISTINCT ON for other backends is currently "
+                "silently ignored, however this usage is deprecated, and will "
+                "raise CompileError in a future release for all backends "
+                "that do not support this syntax.",
+                version="1.4",
+            )
+        return "DISTINCT " if select._distinct else ""
 
     def group_by_clause(self, select, **kw):
         """allow dialects to customize how GROUP BY is rendered."""
diff --git a/lib/sqlalchemy/sql/selectable.py b/lib/sqlalchemy/sql/selectable.py
index 89ee7e28f..c8df637ba 100644
--- a/lib/sqlalchemy/sql/selectable.py
+++ b/lib/sqlalchemy/sql/selectable.py
@@ -4138,6 +4138,9 @@ class Select(
          the PostgreSQL dialect will render a ``DISTINCT ON (<expressions>>)``
          construct.
 
+         .. deprecated:: 1.4 Using \*expr in other dialects is deprecated
+            and will raise :class:`_exc.CompileError` in a future version.
+
         """
         if expr:
             self._distinct = True