is INNER join so much slower than LEFT join ?

[C.R.Vegelin]

Hi List,

I have the 2 MyISAM tables using mySQL version 5.0.15-NT:

Table countries:
`ID` smallint unsigned NOT NULL default '0',
`Code` char(2) default NULL,
`Name` char(30) default NULL, ...
PRIMARY KEY (`ID`)

Table data
`Country1` smallint unsigned NOT NULL default '0',
`Country2` smallint unsigned NOT NULL default '0', ...
KEY `Country1` (`Country1`), KEY `Country2` (`Country2`)

When I run then next query with LEFT join is takes approx 1 minute.
UPDATE data AS db
LEFT JOIN countries AS c1 ON db.Country1=c1.ID
LEFT JOIN countries AS c2 ON db.Country2=c2.ID
SET db.Expr = ...;

But when I run it with INNER join is takes more than 2 hours !!!
In both cases the query applies to 9.571.220 rows matched with 0 changed.
Any idea why INNER join is so much slower ?

TIA, Cor