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Fix bug lp:777691

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Analysis:

For some of the re-executions of the correlated subquery the
where clause is false. In these cases the execution of the
subquery detects that it must generate a NULL row because of
implicit grouping. In this case the subquery execution reaches
the following code in do_select():

        while ((table= li++))
          mark_as_null_row(table->table);

This code marks all rows in the table as complete NULL rows.
In the example, when evaluating the field t2.f10 for the second
row, all bits of Field::null_ptr[0] are set by the previous call
to mark_as_null_row(). Then the call to Field::is_null()
returns true, resulting in a NULL for the MAX function.

Thus the lines above are not suitable for subquery re-execution
because mark_as_null_row() changes the NULL bits of each table
field, and there is no logic to restore these fields.

Solution:

The call to mark_as_null_row() was added by the fix for bug
lp:613029. Therefore removing the fix for lp:613029 corrects
this wrong result. At the same time the test for lp:613029
behaves correctly because the changes of MWL#89 result in a
different execution path where:
- the constant subquery is evaluated via JOIN::exec_const_cond
- detecting that it has an empty result triggers the branch
  if (zero_result_cause)
    return_zero_rows()
- return_zero_rows() calls mark_as_null_row().
This commit is contained in:
unknown 2011-07-14 12:53:00 +03:00
parent c4097382ef
commit 53681ee5db
3 changed files with 82 additions and 8 deletions
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48
mysql-test/r/subselect4.result
View file

@ -1190,6 +1190,54 @@ set @@optimizer_switch='materialization=off,semijoin=on';
set @@optimizer_switch=@save_optimizer_switch;
drop table t1, t2;
#
# LP BUG#777691 Wrong result with subqery in select list and subquery cache=off in maria-5.3
#
CREATE TABLE t1 ( f1 varchar(32)) ;
INSERT INTO t1 VALUES ('b'),('x'),('c'),('x');
CREATE TABLE t2 ( f2 int, f3 varchar(32)) ;
INSERT INTO t2 VALUES (1,'x');
set @save_optimizer_switch=@@optimizer_switch;
set @@optimizer_switch='materialization=off,in_to_exists=on,subquery_cache=off';
EXPLAIN
SELECT t1.f1, ( SELECT MAX( f2 ) FROM t2 WHERE t2.f3 = t1.f1 ) as max_f2 FROM t1;
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY t1 ALL NULL NULL NULL NULL 4
2 DEPENDENT SUBQUERY t2 system NULL NULL NULL NULL 1
SELECT t1.f1, ( SELECT MAX( f2 ) FROM t2 WHERE t2.f3 = t1.f1 ) as max_f2 FROM t1;
f1 max_f2
b NULL
x 1
c NULL
x 1
set @@optimizer_switch='materialization=on,in_to_exists=off,subquery_cache=off';
EXPLAIN
SELECT t1.f1, ( SELECT MAX( f2 ) FROM t2 WHERE t2.f3 = t1.f1 ) as max_f2 FROM t1;
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY t1 ALL NULL NULL NULL NULL 4
2 DEPENDENT SUBQUERY t2 system NULL NULL NULL NULL 1
SELECT t1.f1, ( SELECT MAX( f2 ) FROM t2 WHERE t2.f3 = t1.f1 ) as max_f2 FROM t1;
f1 max_f2
b NULL
x 1
c NULL
x 1
set @@optimizer_switch='materialization=off,in_to_exists=on,subquery_cache=off';
Even when t2 is not constant table, the result must be the same.
INSERT INTO t2 VALUES (2,'y');
EXPLAIN
SELECT t1.f1, ( SELECT MAX( f2 ) FROM t2 WHERE t2.f3 = t1.f1 ) as max_f2 FROM t1;
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY t1 ALL NULL NULL NULL NULL 4
2 DEPENDENT SUBQUERY t2 ALL NULL NULL NULL NULL 2 Using where
SELECT t1.f1, ( SELECT MAX( f2 ) FROM t2 WHERE t2.f3 = t1.f1 ) as max_f2 FROM t1;
f1 max_f2
b NULL
x 1
c NULL
x 1
set @@optimizer_switch=@save_optimizer_switch;
drop table t1, t2;
#
# LP BUG#641203 Query returns rows where no result is expected (impossible WHERE)
#
CREATE TABLE t1 (c1 varchar(1) DEFAULT NULL);

34
mysql-test/t/subselect4.test
View file

@ -964,6 +964,40 @@ WHERE (t2.pk = t1.pk) AND t2.pk IN (SELECT f2 FROM t1);
-- echo set @@optimizer_switch='materialization=off,semijoin=on';
set @@optimizer_switch=@save_optimizer_switch;
drop table t1, t2;
--echo #
--echo # LP BUG#777691 Wrong result with subqery in select list and subquery cache=off in maria-5.3
--echo #
CREATE TABLE t1 ( f1 varchar(32)) ;
INSERT INTO t1 VALUES ('b'),('x'),('c'),('x');
CREATE TABLE t2 ( f2 int, f3 varchar(32)) ;
INSERT INTO t2 VALUES (1,'x');
set @save_optimizer_switch=@@optimizer_switch;
set @@optimizer_switch='materialization=off,in_to_exists=on,subquery_cache=off';
EXPLAIN
SELECT t1.f1, ( SELECT MAX( f2 ) FROM t2 WHERE t2.f3 = t1.f1 ) as max_f2 FROM t1;
SELECT t1.f1, ( SELECT MAX( f2 ) FROM t2 WHERE t2.f3 = t1.f1 ) as max_f2 FROM t1;
set @@optimizer_switch='materialization=on,in_to_exists=off,subquery_cache=off';
EXPLAIN
SELECT t1.f1, ( SELECT MAX( f2 ) FROM t2 WHERE t2.f3 = t1.f1 ) as max_f2 FROM t1;
SELECT t1.f1, ( SELECT MAX( f2 ) FROM t2 WHERE t2.f3 = t1.f1 ) as max_f2 FROM t1;
set @@optimizer_switch='materialization=off,in_to_exists=on,subquery_cache=off';
--echo Even when t2 is not constant table, the result must be the same.
INSERT INTO t2 VALUES (2,'y');
EXPLAIN
SELECT t1.f1, ( SELECT MAX( f2 ) FROM t2 WHERE t2.f3 = t1.f1 ) as max_f2 FROM t1;
SELECT t1.f1, ( SELECT MAX( f2 ) FROM t2 WHERE t2.f3 = t1.f1 ) as max_f2 FROM t1;
set @@optimizer_switch=@save_optimizer_switch;
drop table t1, t2;

8
sql/sql_select.cc
View file

@ -14651,14 +14651,6 @@ do_select(JOIN *join,List<Item> *fields,TABLE *table,Procedure *procedure)
{
List<Item> *columns_list= (procedure ? &join->procedure_fields_list :
fields);
/*
With implicit grouping all fields of special row produced for an
empty result are NULL. See return_zero_rows() for the same behavior.
*/
TABLE_LIST *table;
List_iterator_fast<TABLE_LIST> li(join->select_lex->leaf_tables);
while ((table= li++))
mark_as_null_row(table->table);
rc= join->result->send_data(*columns_list) > 0;
}
}