Remove unnecessary conditions from some queries.

PG9 doesn't like the query from _load_link_comments
in comment_tree.py.

thing.py will automatically filter deleted and spam items
out of thing queries if no rules are specified for those
columns. To get around this, some queries, including the
_load_link_comments one, will specify a rule
like _spam == (True, False). This rule was left in the ruleset,
despite the fact that it's completely unnecessary from the
SQL's standpoint. Its presence then triggered the need of a
join against the thing table when only the data table was
truly necessary. The query planner in PG9 doesn't like this
particular query when it requires a join and will force a
full table scan as a result.

This patch notes the special case of _spam/_deleted ==
(True,False) and turns off the default filtering but removes
the rules from the query. This saves some WHERE clauses
and more importantly removes the need for a JOIN in queries
like this one.

r2/r2/lib/comment_tree.py

@@ -309,6 +309,7 @@ def _load_link_comments(link_id):
     q = Comment._query(Comment.c.link_id == link_id,
                        Comment.c._deleted == (True, False),
                        Comment.c._spam == (True, False),
+                       optimize_rules=True,
                        data = True)
     comments = list(q)
     cids = [c._id for c in comments]

r2/r2/lib/db/thing.py

@@ -585,20 +585,28 @@ class Thing(DataThing):
                    bases.deleted, bases.spam, id)
 
     @classmethod
-    def _query(cls, *rules, **kw):
+    def _query(cls, *all_rules, **kw):
         need_deleted = True
         need_spam = True
         #add default spam/deleted
-        for r in rules:
+        rules = []
+        optimize_rules = kw.pop('optimize_rules', False)
+        for r in all_rules:
             if not isinstance(r, operators.op):
                 continue
             if r.lval_name == '_deleted':
                 need_deleted = False
+                # if the caller is explicitly unfiltering based on this column,
+                # we don't need this rule at all. taking this out can save us a
+                # join that is very expensive on pg9.
+                if optimize_rules and r.rval == (True, False):
+                    continue
             elif r.lval_name == '_spam':
                 need_spam = False
-
-        if need_deleted or need_spam:
-            rules = list(rules)
+                # see above for explanation
+                if optimize_rules and r.rval == (True, False):
+                    continue
+            rules.append(r)
 
         if need_deleted:
             rules.append(cls.c._deleted == False)