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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
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
mysql-test/r/ps_11bugs.result
View File

@ -121,8 +121,8 @@ insert into t1 values (1);
explain select * from t1 where 3 in (select (1+1) union select 1);
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY NULL NULL NULL NULL NULL NULL NULL Impossible WHERE noticed after reading const tables
2 DEPENDENT SUBQUERY NULL NULL NULL NULL NULL NULL NULL No tables used
3 DEPENDENT UNION NULL NULL NULL NULL NULL NULL NULL No tables used
2 DEPENDENT SUBQUERY NULL NULL NULL NULL NULL NULL NULL Impossible HAVING
3 DEPENDENT UNION NULL NULL NULL NULL NULL NULL NULL Impossible HAVING
NULL UNION RESULT <union2,3> ALL NULL NULL NULL NULL NULL
select * from t1 where 3 in (select (1+1) union select 1);
a

4
mysql-test/r/subselect.result
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@ -4353,13 +4353,13 @@ id select_type table type possible_keys key key_len ref rows filtered Extra
1 PRIMARY t1 ALL NULL NULL NULL NULL 2 100.00
2 DEPENDENT SUBQUERY t1 ALL NULL NULL NULL NULL 2 100.00 Using temporary; Using filesort
Warnings:
Note 1003 select 1 AS `1` from `test`.`t1` where <in_optimizer>(1,<exists>(select 1 AS `1` from `test`.`t1` group by `test`.`t1`.`a` having (<cache>(1) = <ref_null_helper>(1))))
Note 1003 select 1 AS `1` from `test`.`t1` where <in_optimizer>(1,<exists>(select 1 AS `1` from `test`.`t1` group by `test`.`t1`.`a` having 1))
EXPLAIN EXTENDED SELECT 1 FROM t1 WHERE 1 IN (SELECT 1 FROM t1 WHERE a > 3 GROUP BY a);
id select_type table type possible_keys key key_len ref rows filtered Extra
1 PRIMARY NULL NULL NULL NULL NULL NULL NULL NULL Impossible WHERE noticed after reading const tables
2 DEPENDENT SUBQUERY t1 ALL NULL NULL NULL NULL 2 100.00 Using where; Using temporary; Using filesort
Warnings:
Note 1003 select 1 AS `1` from `test`.`t1` where <in_optimizer>(1,<exists>(select 1 AS `1` from `test`.`t1` where (`test`.`t1`.`a` > 3) group by `test`.`t1`.`a` having (<cache>(1) = <ref_null_helper>(1))))
Note 1003 select 1 AS `1` from `test`.`t1` where <in_optimizer>(1,<exists>(select 1 AS `1` from `test`.`t1` where (`test`.`t1`.`a` > 3) group by `test`.`t1`.`a` having 1))
DROP TABLE t1;
End of 5.0 tests.
CREATE TABLE t1 (a INT, b INT);

15
mysql-test/r/table_elim.result
View File

@ -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;

9
mysql-test/t/table_elim.test
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@ -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;

4
sql/item.h
View File

@ -2203,6 +2203,10 @@ public:
if (!depended_from)
(*ref)->update_used_tables();
}
bool const_item() const
{
return (*ref)->const_item();
}
table_map not_null_tables() const { return (*ref)->not_null_tables(); }
void set_result_field(Field *field) { result_field= field; }
bool is_result_field() { return 1; }

5
sql/item_sum.cc
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@ -542,11 +542,6 @@ void Item_sum::update_used_tables ()
args[i]->update_used_tables();
used_tables_cache|= args[i]->used_tables();
}
used_tables_cache&= PSEUDO_TABLE_BITS;
/* the aggregate function is aggregated into its local context */
used_tables_cache |= (1 << aggr_sel->join->tables) - 1;
}
}

14
sql/item_sum.h
View File

@ -255,6 +255,12 @@ protected:
*/
Item **orig_args, *tmp_orig_args[2];
table_map used_tables_cache;
/*
TRUE <=> We've managed to calculate the value of this Item in
opt_sum_query(), hence it can be considered constant at all subsequent
steps.
*/
bool forced_const;
public:
@ -341,6 +347,14 @@ public:
virtual const char *func_name() const= 0;
virtual Item *result_item(Field *field)
{ return new Item_field(field); }
/*
Return bitmap of tables that are needed to evaluate the item.
The implementation takes into account the used strategy: items resolved
at optimization phase report 0.
Items that depend on the number of rows only, e.g. COUNT(*) will report
zero, but will still false from const_item().
*/
table_map used_tables() const { return used_tables_cache; }
void update_used_tables ();
void cleanup()