performance - Why MySQL is very slow when using JOIN instead of WHERE? -


i have 2 tables:

create table `test_sample` (   `idtest_sample` varchar(50) not null,   `test_samplecol` varchar(45) default null,   unique key `idtest_sample_unique` (`idtest_sample`) ) engine=myisam default charset=utf8

and

create table `new_table` (   `idnew_table` int(11) not null,   unique key `idnew_table_unique` (`idnew_table`) ) engine=myisam default charset=utf8

the first table contains 5 million records while second 1 10 records.

the duration of execution of query more 5 seconds:

select * test_sample  inner join new_table  on test_sample.idtest_sample = new_table.idnew_table

while query executed (less 0.001 second):

select * test_sample  test_sample.idtest_sample in  ('3','1597','25963','170596','196485', '545963','999999','1265896','1569485','1999999')

why first query takes long?

did try see execution path ? desc {sql}, first 1 longer

the first query entire 5 millions rows each row of other table. while second query specific id have index (as they're part of primary key)

edit: here explain (simplified): 

+----+-------------+-------------+--------+----------------------+------+ | id | select_type | table       | type   | possible_keys        | key  | +----+-------------+-------------+--------+----------------------+------+ |  1 | simple      | new_table   | system | idnew_table_unique   | null | |  1 | simple      | test_sample |    | idtest_sample_unique | null | +----+-------------+-------------+--------+----------------------+------+ +----+-------------+-------------+-------+----------------------+----------------------+ | id | select_type | table       | type  | possible_keys        | key                  | +----+-------------+-------------+-------+----------------------+----------------------+ |  1 | simple      | test_sample | const | idtest_sample_unique | idtest_sample_unique | +----+-------------+-------------+-------+----------------------+----------------------+

as can see there 'all' scan on test_sample table (5 millions line)

you may want take here : http://hackmysql.com/case4


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