Deprecate use of whitespace to split schema/table name

Addresses rails#26721

activerecord/lib/active_record/connection_adapters/postgresql/utils.rb

@@ -69,7 +69,21 @@ def extract_schema_qualified_name(string)
             table = schema
             schema = nil
           end
-          PostgreSQL::Name.new(schema, table)
+
+          name = PostgreSQL::Name.new(schema, table)
+
+          if schema
+            # split happened, did it happen because of spaces in an unquoted string? See #26721
+            if !string.include?(".") && string =~ /\s/
+              ActiveSupport::Deprecation.warn(<<-MSG.squish)
+                Specifying schema with a space is deprecated.
+                Please use dot syntax: #{"'\"#{name.schema}\".\"#{name.identifier}\"'"}
+                To use spaces in a table name, use quotes: #{"'\"#{string}\"'"}
+              MSG
+            end
+          end
+
+          name
         end
       end
     end

activerecord/test/cases/adapters/postgresql/utils_test.rb

@@ -15,10 +15,20 @@ def test_extract_schema_qualified_name
       %("schema"."table_name") => %w{schema table_name},
       %("even spaces".table)   => ["even spaces","table"],
       %(schema."table.name")   => ["schema", "table.name"]
+
     }.each do |given, expect|
-      assert_equal Name.new(*expect), extract_schema_qualified_name(given)
+      assert_not_deprecated {
+        assert_equal Name.new(*expect), extract_schema_qualified_name(given)
+      }
     end
   end
+
+  def test_extract_schema_qualified_name_deprecated
+    assert_deprecated {
+      assert_equal Name.new("schema", "table_name"), extract_schema_qualified_name("schema table_name")
+      assert_equal Name.new("schema", "table_name"), extract_schema_qualified_name("schema  \t table_name")
+    }
+  end
 end
 
 class PostgreSQLNameTest < ActiveRecord::PostgreSQLTestCase