#At file:///export/home/tmp/mysql2/opt-bug59696/ based on revid:anitha.gopi@stripped
3559 Olav Sandstaa 2011-01-27
Fix for Bug#59696 Optimizer fails to move WHERE condition on JOIN column
when joining with a view
When optimizing a JOIN of two tables the optimizer tries to move WHERE
conditions on the join column(s) to the first table in the JOIN in
order to reduce the number of records it has to read. If the second
operand of the JOIN is a view that is identical to the underlying
table the optimizer should be able to perform the same optimization.
In this case it fails to move the WHERE condition on the join column
to the first table.
The cause for this is that when the second operand is a view then the
WHERE clauses is stored using Item_ref object that references the real
item objects. The existing code fails to evaluate these since it only
evaluates the Item_ref object and fails to look at the real
objects. As a result it fails to detect that the WHERE clause on the
second join operand can be substituted by a corresponding WHERE clause
on the first operand.
There are two places where we fail to handle Item_ref correctly and that
causes the optimizer to fail to do this optimization:
1. In optimize_cond() we call build_equal_items() which again calls
build_equal_items_for_cond() where we in Item_func::compile() fail to
evaluate the Item::equal_fields_propagator() for the condition on the
view operand. As a result the field used in the condition for the view
is not considered as an equal field to the join column of the first
table when this later is evaluated in make_join_statistics().
2. Later in JOIN::optimize() we call substitute_for_best_equal_field()
which uses Item_func::transform() to select the best condition of the
tables. Item_func::transform() fails to evaluate the different
conditions that are equal due to these being stored in a Item_ref
object.
The fix for these problems is to implement compile() and transform()
methods for the Item_ref class that will do the proper evaluation of
the compile() and transform on the objects the Item_ref object has the
reference to. This will ensure that when Item_func::compile() and
Item_func::transform() evaluates Item_ref objects both the Item_ref
object and the object it references get evaluated.
@ mysql-test/r/join.result
Test case for Bug#59696 Optimizer fails to move WHERE condition
on JOIN column when joining with a view.
@ mysql-test/r/subquery_sj_none.result
Change in explain/query plan due to the fix for Bug#59696 "Optimizer
fails to move WHERE condition on JOIN column when joining with a view"
The change in query plan for the following query:
select * from t2 where (b,a) in (select a,b from t1 where a=3);
where the table t2 is defined as:
create table t2 as select * from t1;
is caused by that when the optimizer optimizes the subquery it will
try to optimize the conditions (in optimize_cond()). One of the things
it does it to call build_equal_items_for_cond() for the following
condition:
(<cache>(`test`.`t2`.`b`) = `test`.`t1`.`a`)
When this is "compiled" in build_equal_items_for_cond() both the
<cache>(`test`.`t2`.`b`) and `test`.`t1`.`a` are stored in Item_ref
objects. So with the existing code the real items does not get
"compiled" and are not considered for alternatives. So the result of
the "compilation" is:
(<cache>(`test`.`t2`.`b`) = `test`.`t1`.`a`)
With the patch applied both of the real items get evaluated and the
code is able to find that `test`.`t1`.`a` can be replaced by the
constant "3". So the result is from the "compilation" of this condition is
<cache>(`test`.`t2`.`b`) = 3
Which causes the execution plan for the subquery to change as it has.
@ mysql-test/t/join.test
Test case for Bug#59696 Optimizer fails to move WHERE condition
on JOIN column when joining with a view.
@ sql/item.cc
Implement transform() and compile() methods for the Item_ref class.
These methods will then do transform/compile for both the
Item_ref object and on the object it has a reference to.
@ sql/item.h
Implement transform() and compile() methods for the Item_ref class.
modified:
mysql-test/r/join.result
mysql-test/r/subquery_sj_none.result
mysql-test/t/join.test
sql/item.cc
sql/item.h
=== modified file 'mysql-test/r/join.result'
=== modified file 'mysql-test/r/join.result'
--- a/mysql-test/r/join.result 2010-12-16 17:38:26 +0000
+++ b/mysql-test/r/join.result 2011-01-27 15:51:29 +0000
@@ -1222,3 +1222,40 @@
DEALLOCATE PREPARE stmt;
DROP TABLE t1;
End of 5.1 tests
+#
+# Bug #59696 Optimizer fails to move WHERE condition on JOIN column
+# when joining with a view
+#
+CREATE TABLE t1 (
+c1 INTEGER NOT NULL
+);
+INSERT INTO t1 VALUES (1),(2),(3);
+CREATE TABLE t2 (
+pk INTEGER NOT NULL,
+c1 INTEGER NOT NULL,
+PRIMARY KEY (pk)
+);
+INSERT INTO t2 VALUES (1,4),(3,5),(2,6);
+EXPLAIN SELECT t2.pk, t2.c1 FROM t2, t1
+WHERE t2.pk = t1.c1 AND t2.pk >= 2;
+id select_type table type possible_keys key key_len ref rows Extra
+1 SIMPLE t1 ALL NULL NULL NULL NULL 3 Using where
+1 SIMPLE t2 eq_ref PRIMARY PRIMARY 4 test.t1.c1 1
+SELECT t2.pk, t2.c1 FROM t2, t1
+WHERE t2.pk = t1.c1 AND t2.pk >= 2;
+pk c1
+2 6
+3 5
+CREATE VIEW v_t2 AS SELECT * FROM t2;
+EXPLAIN SELECT v_t2.pk, v_t2.c1 FROM v_t2, t1
+WHERE v_t2.pk = t1.c1 AND v_t2.pk >= 2;
+id select_type table type possible_keys key key_len ref rows Extra
+1 SIMPLE t1 ALL NULL NULL NULL NULL 3 Using where
+1 SIMPLE t2 eq_ref PRIMARY PRIMARY 4 test.t1.c1 1
+SELECT v_t2.pk, v_t2.c1 FROM v_t2, t1
+WHERE v_t2.pk = t1.c1 AND v_t2.pk >= 2;
+pk c1
+2 6
+3 5
+DROP VIEW v_t2;
+DROP TABLE t1, t2;
=== modified file 'mysql-test/r/subquery_sj_none.result'
--- a/mysql-test/r/subquery_sj_none.result 2011-01-13 10:48:28 +0000
+++ b/mysql-test/r/subquery_sj_none.result 2011-01-27 15:51:29 +0000
@@ -2543,7 +2543,7 @@
explain select * from t2 where (b,a) in (select a,b from t1 where a=3);
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY t2 ALL NULL NULL NULL NULL 100 Using where
-2 DEPENDENT SUBQUERY t1 index_subquery a a 10 func,func 1 Using index; Using where
+2 DEPENDENT SUBQUERY t1 ref a a 10 const,func 1 Using where; Using index
drop table t1,t2;
create table t1 (a int, b int);
insert into t1 select a,a from t0;
=== modified file 'mysql-test/t/join.test'
--- a/mysql-test/t/join.test 2010-12-16 17:38:26 +0000
+++ b/mysql-test/t/join.test 2011-01-27 15:51:29 +0000
@@ -934,3 +934,43 @@
DROP TABLE t1;
--echo End of 5.1 tests
+
+--echo #
+--echo # Bug #59696 Optimizer fails to move WHERE condition on JOIN column
+--echo # when joining with a view
+--echo #
+
+CREATE TABLE t1 (
+ c1 INTEGER NOT NULL
+);
+
+INSERT INTO t1 VALUES (1),(2),(3);
+
+CREATE TABLE t2 (
+ pk INTEGER NOT NULL,
+ c1 INTEGER NOT NULL,
+ PRIMARY KEY (pk)
+);
+
+INSERT INTO t2 VALUES (1,4),(3,5),(2,6);
+
+let $query=
+SELECT t2.pk, t2.c1 FROM t2, t1
+WHERE t2.pk = t1.c1 AND t2.pk >= 2;
+
+eval EXPLAIN $query;
+eval $query;
+
+# Create a view on one of the tables. The same query plan should
+# be used when joining with this view as with the underlying table.
+CREATE VIEW v_t2 AS SELECT * FROM t2;
+
+let $query=
+SELECT v_t2.pk, v_t2.c1 FROM v_t2, t1
+WHERE v_t2.pk = t1.c1 AND v_t2.pk >= 2;
+
+eval EXPLAIN $query;
+eval $query;
+
+DROP VIEW v_t2;
+DROP TABLE t1, t2;
=== modified file 'sql/item.cc'
--- a/sql/item.cc 2011-01-18 10:32:38 +0000
+++ b/sql/item.cc 2011-01-27 15:51:29 +0000
@@ -6518,6 +6518,85 @@
}
+/**
+ Transform an Item_ref object with a transformer callback function.
+
+ The function first applies the transform method to the item
+ referenced by this Item_reg object. If this returns a new item the
+ old item is substituted for a new one. After this the transformer
+ is applied to the Item_ref object.
+
+ @param transformer the transformer callback function to be applied to
+ the nodes of the tree of the object
+ @param argument parameter to be passed to the transformer
+
+ @return Item returned as the result of transformation of the Item_ref object
+ @retval !NULL The transformation was successful
+ @retval NULL Out of memory error
+*/
+
+Item* Item_ref::transform(Item_transformer transformer, uchar *arg)
+{
+ DBUG_ASSERT(!current_thd->is_stmt_prepare());
+ DBUG_ASSERT((*ref) != NULL);
+
+ /* Transform the object we are referencing. */
+ Item *new_item= (*ref)->transform(transformer, arg);
+ if (!new_item)
+ return NULL;
+
+ /*
+ THD::change_item_tree() should be called only if the tree was
+ really transformed, i.e. when a new item has been created.
+ Otherwise we'll be allocating a lot of unnecessary memory for
+ change records at each execution.
+ */
+ if (*ref != new_item)
+ current_thd->change_item_tree(ref, new_item);
+
+ /* Transform the item ref object. */
+ return (this->*transformer)(arg);
+}
+
+
+/**
+ Compile an Item_ref object with a processor and a transformer
+ callback functions.
+
+ First the function applies the analyzer to the Item_ref object. Then
+ if the analizer succeeeds we first applies the compile method to the
+ object the Item_ref object is referencing. If this returns a new
+ item the old item is substituted for a new one. After this the
+ transformer is applied to the Item_ref object itself.
+
+ @param analyzer the analyzer callback function to be applied to the
+ nodes of the tree of the object
+ @param[in,out] arg_p parameter to be passed to the processor
+ @param transformer the transformer callback function to be applied to the
+ nodes of the tree of the object
+ @param arg_t parameter to be passed to the transformer
+
+ @return Item returned as the result of transformation of the Item_ref object
+*/
+
+Item* Item_ref::compile(Item_analyzer analyzer, uchar **arg_p,
+ Item_transformer transformer, uchar *arg_t)
+{
+ /* Analyze this Item object. */
+ if (!(this->*analyzer)(arg_p))
+ return NULL;
+
+ /* Compile the Item we are referencing. */
+ DBUG_ASSERT((*ref) != NULL);
+ Item *new_item= (*ref)->compile(analyzer, arg_p, transformer, arg_t);
+ if (new_item && *ref != new_item)
+ current_thd->change_item_tree(ref, new_item);
+
+ /* Transform this Item object. */
+ return (this->*transformer)(arg_t);
+}
+
+
void Item_ref::print(String *str, enum_query_type query_type)
{
if (ref)
=== modified file 'sql/item.h'
--- a/sql/item.h 2011-01-10 16:37:47 +0000
+++ b/sql/item.h 2011-01-27 15:51:29 +0000
@@ -2620,6 +2620,9 @@
return (*ref)->walk(processor, walk_subquery, arg) ||
(this->*processor)(arg);
}
+ virtual Item* transform(Item_transformer, uchar *arg);
+ virtual Item* compile(Item_analyzer analyzer, uchar **arg_p,
+ Item_transformer transformer, uchar *arg_t);
virtual void print(String *str, enum_query_type query_type);
bool result_as_longlong()
{
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