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MWL#17: Table elimination

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- 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
This commit is contained in:
Sergey Petrunia
2009-06-10 01:11:33 +04:00
parent cf028a5474
commit fd485ad988
7 changed files with 44 additions and 11 deletions
Download Patch File Download Diff File Expand all files Collapse all files

15
mysql-test/r/table_elim.result
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@ -53,4 +53,19 @@ t3.a=t1.a) on t0.a=t1.a;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t0 ALL NULL NULL NULL NULL 4
1 SIMPLE t1 ALL NULL NULL NULL NULL 4
# Elimination with aggregate functions
explain select count(*) from t1 left join t2 on t2.a=t1.a;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 ALL NULL NULL NULL NULL 4
explain select count(1) from t1 left join t2 on t2.a=t1.a;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 ALL NULL NULL NULL NULL 4
explain select count(1) from t1 left join t2 on t2.a=t1.a group by t1.a;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 ALL NULL NULL NULL NULL 4 Using temporary; Using filesort
This must not use elimination:
explain select count(1) from t1 left join t2 on t2.a=t1.a group by t2.a;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 ALL NULL NULL NULL NULL 4 Using temporary; Using filesort
1 SIMPLE t2 eq_ref PRIMARY PRIMARY 4 test.t1.a 1 Using index
drop table t0, t1, t2, t3;