Work around table_exists? issue on derby with new SELECT NULL syntax

Derby must do some aggressive optimization, where if you don't actually
need the table, it doesn't even check if it exists.  So refactor the
default handling to make it easy to override, and override it for
derby.

lib/sequel/adapters/jdbc/derby.rb

@@ -37,6 +37,12 @@ def svn_version
 
         private
 
+        # Derby optimizes away Sequel's default check of SELECT NULL FROM table,
+        # so use a SELECT * FROM table there.
+        def _table_exists?(ds)
+          ds.first
+        end
+
         # Derby-specific syntax for renaming columns and changing a columns type/nullity.
         def alter_table_sql(table, op)
           case op[:op]

lib/sequel/database/query.rb

@@ -191,12 +191,12 @@ def schema(table, opts={})
     def table_exists?(name)
       sch, table_name = schema_and_table(name)
       name = SQL::QualifiedIdentifier.new(sch, table_name) if sch
-      from(name).get(Sequel::NULL)
+      _table_exists?(from(name))
       true
     rescue DatabaseError
       false
     end
-    
+
     # Return all tables in the database as an array of symbols.
     #
     #   DB.tables # => [:albums, :artists]
@@ -241,6 +241,12 @@ def views(opts={})
 
     private
 
+    # Should raise an error if the table doesn't not exist,
+    # and not raise an error if the table does exist.
+    def _table_exists?(ds)
+      ds.get(Sequel::NULL)
+    end
+
     # Internal generic transaction method.  Any exception raised by the given
     # block will cause the transaction to be rolled back.  If the exception is
     # not a Sequel::Rollback, the error will be reraised. If no exception occurs