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

1 files changed, 7 insertions, 0 deletions

diff --git a/test/sql/test_compiler.py b/test/sql/test_compiler.py
index e44deed90..440eaa05a 100644
--- a/test/sql/test_compiler.py
+++ b/test/sql/test_compiler.py
@@ -1462,6 +1462,13 @@ class SelectTest(fixtures.TestBase, AssertsCompiledSQL):
             "SELECT count(DISTINCT mytable.myid) AS count_1 FROM mytable",
         )
 
+    def test_distinct_on(self):
+        with testing.expect_deprecated(
+            "DISTINCT ON is currently supported only by the PostgreSQL "
+            "dialect"
+        ):
+            select(["*"]).distinct(table1.c.myid).compile()
+
     def test_where_empty(self):
         self.assert_compile(
             select([table1.c.myid]).where(