Wrong result for aggregate function with distinct clause when the value for tmp_table_size is small

create table t1 (

a VARCHAR(900),

b int

);

insert into t1 values

( 0 , 1 ),

( 1 , 2 ),

( 2 , 3 ),

( 3 , 4 ),

( 4 , 5 ),

( 5 , 6 ),

( 6 , 7 ),

( 7 , 8 ),

( 8 , 9 ),

( 9 , 10 ),

( 10 , 11 ),

( 11 , 12 ),

( 12 , 13 ),

( 13 , 14 ),

( 14 , 15 ),

( 15 , 16 ),

( 16 , 17 ),

( 17 , 18 ),

( 18 , 19 ),

( 25 , 26 ),

( 26 , 27 ),

( 9 , 120 ),

( 10 , 121 ),

( 11 , 122 ),

( 12 , 123 ),

( 13 , 124 ),

( 14 , 125 ),

( 15 , 126 ),

( 16 , 127 );

select count(distinct a) from t1;

count(distinct a)

21

set tmp_table_size=1024;

select count(distinct a) from t1;

count(distinct a)

19