added module to contain optimizations

git-svn-id: http://galileo.dmi.unict.it/svn/relational/trunk@134 014f5005-505e-4b48-8d0a-63407b615a7c

relational/optimizations.py

@@ -0,0 +1,28 @@
+# -*- coding: utf-8 -*-
+# Relational
+# Copyright (C) 2009  Salvo "LtWorf" Tomaselli
+# 
+# Relation is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+# 
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+# 
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+# 
+# author Salvo "LtWorf" Tomaselli <tiposchi@tiscali.it>
+
+'''This module contains functions to perform various optimizations on the expression trees.
+The list general_optimizations contains pointers to general functions, so they can be called
+within a cycle.'''
+
+def duplicated_select(n):
+    pass
+
+
+general_optimizations=[duplicated_select]

relational/optimizer.py

@@ -20,6 +20,9 @@
 
 '''This module optimizes relational expressions into ones that require less time to be executed
 For now it is highly experimental, and it shouldn't be used in 3rd party applications.'''
+
+import optimizations
+
 RELATION=0
 UNARY=1
 BINARY=2
@@ -165,12 +168,14 @@ def tokenize(expression):
 def tree(expression):
     '''This function parses a relational algebra expression into a tree and returns
     the root node using the Node class defined in this module.'''
-    #isinstance(k,list)
     return node(tokenize(expression))
 
-def optimize(expression):
+def general_optimize(expression):
+    '''This function performs general optimizations. Means that it will not need to
+    know the fields used by the relations'''
     n=tree(expression) #Gets the tree
-    
+    for i in optimizations.general_optimizations:
+        n=i(n) #Performs the optimization
     return n.__str__()
 
 if __name__=="__main__":