updated CHANGELOG for #2071

CHANGELOG

@@ -3,6 +3,39 @@ devel
 
 * fixed issue #2071
 
+  make the AQL query optimizer inject filter condition expressions referred to by variables during filter condition aggregation.
+  for example, in the following query
+  
+      FOR doc IN collection
+	LET cond1 = (doc.value == 1)
+	LET cond2 = (doc.value == 2)
+	FILTER cond1 || cond2
+	RETURN { doc, cond1, cond2 }
+  
+  the optimizer will now inject the conditions for `cond1` and `cond2` into the filter condition `cond1 || cond2`, expanding it to 
+  `(doc.value == 1) || (doc.value == 2)` and making these conditions available for index searching.
+
+  note that the optimizer previously already injected some conditions into other conditions, but only if the variable that defined 
+  the condition was not used elsewhere. for example, the filter condition in the query
+  
+      FOR doc IN collection
+	LET cond = (doc.value == 1)
+	FILTER cond
+	RETURN { doc }
+  
+  already got optimized before because `cond` was only used once in the query and the optimizer decided to inject it into the 
+  place where it was used.
+  
+  this only worked for variables that were referred to once in the query. when a variable was used multiple times, the condition 
+  was not injected as in the following query
+
+      FOR doc IN collection
+	LET cond = (doc.value == 1)
+	FILTER cond
+	RETURN { doc, cond }
+
+  the fix for #2070 now will enable this optimization so that the query can use an index on `doc.value` if available.
+
 * changed behavior of AQL array comparison operators for empty arrays:
   * `ALL` and `ANY` now always returns `false` when the left-hand operand is an
     empty array. The behavior for non-empty arrays does not change: