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

3 files changed, 20 insertions, 1 deletions

diff --git a/lib/sqlalchemy/testing/assertions.py b/lib/sqlalchemy/testing/assertions.py
index 61d272160..05dcf230b 100644
--- a/lib/sqlalchemy/testing/assertions.py
+++ b/lib/sqlalchemy/testing/assertions.py
@@ -412,7 +412,6 @@ class AssertsCompiledSQL(object):
         c = clause.compile(dialect=dialect, **kw)
 
         param_str = repr(getattr(c, "params", {}))
-
         if util.py3k:
             param_str = param_str.encode("utf-8").decode("ascii", "ignore")
             print(
diff --git a/lib/sqlalchemy/testing/requirements.py b/lib/sqlalchemy/testing/requirements.py
index 644483b79..31011a970 100644
--- a/lib/sqlalchemy/testing/requirements.py
+++ b/lib/sqlalchemy/testing/requirements.py
@@ -1170,6 +1170,11 @@ class SuiteRequirements(Requirements):
         return exclusions.closed()
 
     @property
+    def supports_distinct_on(self):
+        """If a backend supports the DISTINCT ON in a select"""
+        return exclusions.closed()
+
+    @property
     def supports_is_distinct_from(self):
         """Supports some form of "x IS [NOT] DISTINCT FROM y" construct.
         Different dialects will implement their own flavour, e.g.,
diff --git a/lib/sqlalchemy/testing/suite/test_select.py b/lib/sqlalchemy/testing/suite/test_select.py
index c1f77a7c1..ad17ebb4a 100644
--- a/lib/sqlalchemy/testing/suite/test_select.py
+++ b/lib/sqlalchemy/testing/suite/test_select.py
@@ -12,6 +12,7 @@ from ..schema import Column
 from ..schema import Table
 from ... import bindparam
 from ... import case
+from ... import column
 from ... import Computed
 from ... import false
 from ... import func
@@ -20,6 +21,7 @@ from ... import literal_column
 from ... import null
 from ... import select
 from ... import String
+from ... import table
 from ... import testing
 from ... import text
 from ... import true
@@ -1016,6 +1018,19 @@ class ComputedColumnTest(fixtures.TablesTest):
             eq_(res, [(100, 40), (1764, 168)])
 
 
+class DistinctOnTest(AssertsCompiledSQL, fixtures.TablesTest):
+    __backend__ = True
+    __requires__ = ("standard_cursor_sql",)
+
+    @testing.fails_if(testing.requires.supports_distinct_on)
+    def test_distinct_on(self):
+        stm = select(["*"]).distinct(column("q")).select_from(table("foo"))
+        with testing.expect_deprecated(
+            "DISTINCT ON is currently supported only by the PostgreSQL "
+        ):
+            self.assert_compile(stm, "SELECT DISTINCT * FROM foo")
+
+
 class IsOrIsNotDistinctFromTest(fixtures.TablesTest):
     __backend__ = True
     __requires__ = ("supports_is_distinct_from",)