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

4 files changed, 18 insertions, 20 deletions

diff --git a/lib/sqlalchemy/dialects/firebird/base.py b/lib/sqlalchemy/dialects/firebird/base.py
index a43413780..5779ac885 100644
--- a/lib/sqlalchemy/dialects/firebird/base.py
+++ b/lib/sqlalchemy/dialects/firebird/base.py
@@ -525,8 +525,7 @@ class FBCompiler(sql.compiler.SQLCompiler):
             result += "FIRST %s " % self.process(select._limit_clause, **kw)
         if select._offset_clause is not None:
             result += "SKIP %s " % self.process(select._offset_clause, **kw)
-        if select._distinct:
-            result += "DISTINCT "
+        result += super(FBCompiler, self).get_select_precolumns(select, **kw)
         return result
 
     def limit_clause(self, select, **kw):
diff --git a/lib/sqlalchemy/dialects/mssql/base.py b/lib/sqlalchemy/dialects/mssql/base.py
index df6196bae..0dd2ac11b 100644
--- a/lib/sqlalchemy/dialects/mssql/base.py
+++ b/lib/sqlalchemy/dialects/mssql/base.py
@@ -1636,9 +1636,7 @@ class MSSQLCompiler(compiler.SQLCompiler):
     def get_select_precolumns(self, select, **kw):
         """ MS-SQL puts TOP, it's version of LIMIT here """
 
-        s = ""
-        if select._distinct:
-            s += "DISTINCT "
+        s = super(MSSQLCompiler, self).get_select_precolumns(select, **kw)
 
         if select._simple_int_limit and (
             select._offset_clause is None
@@ -1649,12 +1647,8 @@ class MSSQLCompiler(compiler.SQLCompiler):
             # so have to use literal here.
             kw["literal_execute"] = True
             s += "TOP %s " % self.process(select._limit_clause, **kw)
-        if s:
-            return s
-        else:
-            return compiler.SQLCompiler.get_select_precolumns(
-                self, select, **kw
-            )
+
+        return s
 
     def get_from_hint_text(self, table, text):
         return text
diff --git a/lib/sqlalchemy/dialects/mysql/base.py b/lib/sqlalchemy/dialects/mysql/base.py
index 53c916304..38f3fa611 100644
--- a/lib/sqlalchemy/dialects/mysql/base.py
+++ b/lib/sqlalchemy/dialects/mysql/base.py
@@ -1450,19 +1450,22 @@ class MySQLCompiler(compiler.SQLCompiler):
     def get_select_precolumns(self, select, **kw):
         """Add special MySQL keywords in place of DISTINCT.
 
-        .. note::
-
-          this usage is deprecated.  :meth:`_expression.Select.prefix_with`
-          should be used for special keywords at the start
-          of a SELECT.
+        .. deprecated 1.4:: this usage is deprecated.
+           :meth:`_expression.Select.prefix_with` should be used for special
+           keywords at the start of a SELECT.
 
         """
         if isinstance(select._distinct, util.string_types):
+            util.warn_deprecated(
+                "Sending string values for 'distinct' is deprecated in the "
+                "MySQL dialect and will be removed in a future release.  "
+                "Please use :meth:`.Select.prefix_with` for special keywords "
+                "at the start of a SELECT statement",
+                version="1.4",
+            )
             return select._distinct.upper() + " "
-        elif select._distinct:
-            return "DISTINCT "
-        else:
-            return ""
+
+        return super(MySQLCompiler, self).get_select_precolumns(select, **kw)
 
     def visit_join(self, join, asfrom=False, from_linter=None, **kwargs):
         if from_linter:
diff --git a/lib/sqlalchemy/dialects/postgresql/base.py b/lib/sqlalchemy/dialects/postgresql/base.py
index 20540ac02..ca2f6a8a4 100644
--- a/lib/sqlalchemy/dialects/postgresql/base.py
+++ b/lib/sqlalchemy/dialects/postgresql/base.py
@@ -1693,6 +1693,8 @@ class PGCompiler(compiler.SQLCompiler):
         return "ONLY " + sqltext
 
     def get_select_precolumns(self, select, **kw):
+        # Do not call super().get_select_precolumns because
+        # it will warn/raise when distinct on is present
         if select._distinct or select._distinct_on:
             if select._distinct_on:
                 return (