Improved futile_union_intersection_subtraction in case of A-A, when A is a sub-query

git-svn-id: http://galileo.dmi.unict.it/svn/relational/trunk@208 014f5005-505e-4b48-8d0a-63407b615a7c

CHANGELOG

@@ -90,4 +90,5 @@
 - Selection can now accept expressions with parenthesis
 
 0.11
-- Font is set only on windows (Rev 206)
+- Font is set only on windows (Rev 206)
+- Improved futile_union_intersection_subtraction in case of A-A, when A is a sub-query (Rev 208)

relational/optimizations.py

@@ -81,8 +81,11 @@ def duplicated_select(n):
 
 def futile_union_intersection_subtraction(n):
     '''This function locates things like r ᑌ r, and replaces them with r.
-    σ k (r) ᑌ r with r
-    σ k (r) ᑎ r with σ k (r)
+    R ᑌ R  --> R 
+    R ᑎ R --> R 
+    R - R --> σ False (R) 
+    σ k (R) ᑌ R --> R 
+    σ k (R) ᑎ R --> σ k (R)
     '''
     
     changes=0
@@ -109,7 +112,7 @@ def futile_union_intersection_subtraction(n):
         n.kind=optimizer.UNARY
         n.name='σ'
         n.prop='False'
-        n.child=n.left
+        n.child=n.left.get_first_leaf()
         #n.left=n.right=None
 
     return changes+recoursive_scan(futile_union_intersection_subtraction,n)

relational/parser.py

@@ -105,6 +105,16 @@ class node (object):
         else:
             return self.name
         pass
+    def get_first_leaf(self):
+        '''This function returns the most left random leaf in the tree. It is needed by some optimizations.'''
+        if self.kind==RELATION:
+            return self
+        elif self.kind==UNARY:
+            return self.child.get_first_leaf()
+        elif self.kind==BINARY:
+            return self.left.get_first_leaf()
+        
+        
     def result_format(self,rels):
         '''This function returns a list containing the fields that the resulting relation will have.
         Since it needs to know real instances of relations, it requires a dictionary where keys are