Why would this query run so slow? - mysql

Question

I have two MySQL tables say A and B. A contains just one varchar column (lets call that one A1) with about 23000 records. Table B (70000 records) has some more columns, one of the corresponding with A1 from table A (lets call that one B1). I want to know which values in A are not in the corresponding column in B, so I use:
SELECT A1
FROM A
LEFT JOIN B
ON A1 = B1
WHERE B1 IS NULL
Both columns A1 and B1 have indices defined on them. Still this query runs very slow. I've run explain, this is the output:
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE A index \N PRIMARY 767 \N 23269 Using index
1 SIMPLE B ALL \N \N \N \N 70041 Using where; Not exists
UPDATE: SHOW CREATE TABLE for both tables (changed the original names);
CREATE TABLE `A` (
`A1` varchar(255) NOT NULL,
PRIMARY KEY (`A1`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8
CREATE TABLE `B` (
`col1` int(10) unsigned NOT NULL auto_increment,
`col2` datetime NOT NULL,
`col3` datetime default NULL,
`col4` datetime NOT NULL,
`col5` varchar(30) NOT NULL,
`col6` int(10) default NULL,
`col7` int(11) default NULL,
`col8` varchar(20) NOT NULL,
`B1` varchar(255) default NULL,
`col10` tinyint(1) NOT NULL,
`col11` varchar(255) default NULL,
PRIMARY KEY (`col1`),
KEY `NewIndex1` (`B1`)
) ENGINE=MyISAM AUTO_INCREMENT=70764 DEFAULT CHARSET=latin1
'nother edit: data_length and index_length from SHOW TABLE STATUS
table data_length index_length
A 465380 435200
B 5177996 1344512

Answer 1

The character sets of the two columns that you are comparing in an OUTER JOIN differ. I am not sure if this is the cause so I tested and got these results:
SELECT A1
FROM A
LEFT JOIN B ON A1 = B1
WHERE B1 IS NULL
-- Table A..: 23258 rows, collation = utf8_general_ci
-- Table B..: 70041 rows, collation = latin1_swedish_ci
-- Time ....: I CANCELLED THE QUERY AFTER 20 MINUTES
-- Table A..: 23258 rows, collation = latin1_swedish_ci
-- Table B..: 70041 rows, collation = latin1_swedish_ci
-- Time ....: 0.187 sec
-- Table A..: 23258 rows, collation = utf8_general_ci
-- Table B..: 70041 rows, collation = utf8_general_ci
-- Time ....: 0.344 sec
Solution: make the character sets of the two tables (or the two columns atleast) same.

Answer 2

This query will scan all rows of table A, but if you have an index on B1 then most likely it will not scan table B:
select A1
from A
where not exists (
select *
from B
where B.B1 = A.A1
)
Before running this or your original query you may try to run ANALYZE TABLE in order to update key distribution information for those tables:
ANALYZE TABLE A, B
If this doesn't help then you can try to play with indexes, for instance:
select A1
from A ignore index (PRIMARY)
where not exists (
select *
from B force index (NewIndex1)
where B.B1 = A.A1
)

Answer 3

It seems A1 and B1 are large feilds.
You created indices for both A1 and B1
Make sure that they are indexed!
SELECT A1
FROM A
WHERE A1 NOT IN (
SELECT B1 AS A1 From B;
)

Answer 4

If I use your CREATE TABLES statements and run an EXPLAIN on your SELECT statement, I get this result:
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE A index NULL PRIMARY 767 NULL 2 Using index
1 SIMPLE B index NULL NewIndex1 258 NULL 4 Using where; Using index
On my MySQL version (5.1.41) the index is used as expected, so I think this might be an already fixed bug in MySQL assuming your index is set like in your create table statement posted. What MySQL version do you use?

Answer 5

try this query:
SELECT B1
FROM B
WHERE not B1 in (
select A1
from a
)