MWL#17: Table elimination

- Make elimination work with aggregate functions. The problem was that aggregate functions 
  reported all table bits in used_tables(), and that prevented table elimination. Fixed by 
  making aggregate functions return more correct value from used_tables(). 

mysql-test/r/ps_11bugs.result:
  MWL#17: Table elimination
  - Update test results. The difference is because of Item_ref change: outer references to constants
    are now recognized as constants, too.
mysql-test/r/subselect.result:
  - Update test results. The difference is because of Item_ref change: outer references to constants
    are now recognized as constants, too.
mysql-test/r/table_elim.result:
  MWL#17: Table elimination
  - Check that elimination works in presense of aggreagate functions
mysql-test/t/table_elim.test:
  MWL#17: Table elimination
  - Check that elimination works in presense of aggreagate functions
sql/item.h:
  MWL#17: Table elimination
  - Add Item_ref::const_item() which calls (*ref)->const_item(). Before this diff Item_ref used the 
    default implementation of const_item(){ return used_tables()==0; }. This is no longer true, as 
    COUNT(*) now has used_tables()==0 but const_item()==FALSE.
sql/item_sum.cc:
  MWL#17: Table elimination
  - Make Item_sum() and it descendants not to return all bits in used_tables(). This is needed 
    because otherwise table elimination can't work in presense of aggregate functions
  - COUNT(*) now has used_tables()==0 and const_item()==FALSE. Had to change 
    Item_ref::const_item() to account for this.
sql/item_sum.h:
  MWL#17: Table elimination
  - Add comments

1 files changed, 7 insertions, 2 deletions

diff --git a/mysql-test/t/table_elim.test b/mysql-test/t/table_elim.test
index c540db884ea..4c8c360ea04 100644
--- a/mysql-test/t/table_elim.test
+++ b/mysql-test/t/table_elim.test
@@ -29,8 +29,6 @@ explain select t1.a from t1 left join t2 on t2.a=t1.a order by t2.b;
 --echo # This will not be eliminated as t2.b is in group list:
 explain select t1.a from t1 left join t2 on t2.a=t1.a group by t2.b;
 
-## TODO: Aggregate functions prevent table elimination ATM.
-
 --echo # This will not be eliminated as t2.b is in the WHERE
 explain select t1.a from t1 left join t2 on t2.a=t1.a where t2.b < 3 or t2.b is null;
 
@@ -47,6 +45,13 @@ from
   t0 left join (t1 left join (t2 join t3 on t2.b=t3.b) on t2.a=t1.a and
   t3.a=t1.a) on t0.a=t1.a;
 
+--echo # Elimination with aggregate functions
+explain select count(*) from t1 left join t2 on t2.a=t1.a;
+explain select count(1) from t1 left join t2 on t2.a=t1.a;
+explain select count(1) from t1 left join t2 on t2.a=t1.a group by t1.a;
+
+--echo This must not use elimination:
+explain select count(1) from t1 left join t2 on t2.a=t1.a group by t2.a;
 
 drop table t0, t1, t2, t3;