I doubt that EXPLAIN will ever make any sense/would work reliably with CONNECT tables at al (though it may depend on the type of data source)l. Consequently I think that it would be better to return a(n generic) error message like "EXPLAIN is not available for the actual storage engine" (or a warning at least like "EXPLAIN may return misleading information with storage engines mapping external data sources due to non-availability of internals of the data source").
(I use the term *generic* because it may apply to other storage engines as well (I am having Cassandra and Federated/Federated_x engines in my thoughts too).) -- Peter On Wed, Mar 26, 2014 at 8:44 PM, Sergei Golubchik <[email protected]> wrote: > Hi, Federico! > > On Mar 23, Federico Razzoli wrote: > > Hello list > > > > I found out that EXPLAIN SELECT aggregate_func() FROM connect_table > returns a "rows" value which is much greater than the rows in the table. > For example: > > > > MariaDB [test]> SELECT COUNT(*) FROM t; > > +----------+ > > | COUNT(*) | > > +----------+ > > | 1000000 | > > +----------+ > > 1 row in set (0.49 sec) > > > > MariaDB [test]> EXPLAIN SELECT COUNT(*) FROM t; > > > +------+-------------+-------+------+---------------+------+---------+------+---------+-------+ > > | id | select_type | table | type | possible_keys | key | key_len | > ref | rows | Extra | > > > +------+-------------+-------+------+---------------+------+---------+------+---------+-------+ > > | 1 | SIMPLE | t | ALL | NULL | NULL | NULL | > NULL | 7783705 | | > > > +------+-------------+-------+------+---------------+------+---------+------+---------+-------+ > > > > So, my question is: is there any difference in the output of EXPLAIN > > with CONNECT? Is CONNECT really reading 7.8 millions rows in a table > > with only 1 million rows? > > No, most probably it means that CONNECT estimates in this case are way > off. Could you show a complete test case for this? > > Regards, > Sergei > > > _______________________________________________ > Mailing list: https://launchpad.net/~maria-discuss > Post to : [email protected] > Unsubscribe : https://launchpad.net/~maria-discuss > More help : https://help.launchpad.net/ListHelp >
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