use mysql;

--disable_warnings
drop table if exists t1, t2;
--enable_warnings
create table t1
(
  b varchar(30)
) ENGINE = MYISAM ;
create table t2
(
  b varchar(30)
) ENGINE = BDB ;
commit ;

set @arg00='two' ;

###### statements for comparison purposes
### case 1: outer table(empty), inner table(empty), variable
select 1 as my_column FROM t2 where
exists ( select 1 from t1 where b = @arg00 ) ;

insert into t1 values ('one');
commit;
### case 2: outer table(empty), inner table(non empty), variable
select 1 as my_column FROM t2 where
exists ( select 1 from t1 where b = @arg00 ) ;

### case 3: outer table(non empty), inner table(empty), no variable
select 1 as my_column FROM t1 where
exists ( select 1 from t2 where b = 'two' ) ;

### problem : outer table(non empty), inner table(empty), variable
select 1 as my_column FROM t1 where
exists ( select 1 from t2 where b = @arg00 ) ;

# The (incomplete) property matrix
# remark     outer table inner table  use of variable  problem
# case 1:        empty      empty           YES           NO
# case 2:        empty    non empty         YES           NO
# case 3:     non empty     empty            NO           NO
# problem :   non empty     empty           YES          YES

# BTW: The effect will not change if 
#      - I replace t1 with t2 (work on the same physical table)
#  or  - the constant 1 is replaced with a column from the outer table.

# My assumptions:
# - The problem occurs during execution of the statement, when the server
#   already knows that that the outer table is not empty.
# - The behaviour is very strange, because it depends on the use of the
#   variable. 
# - The problem depends on the use of the BDB table handler for the table t2.
#   If I replace BDB with MYISAM, the problem disappears.