maria-discuss team mailing list archive

[Peter Laursen <peter_laursen@xxxxxxxxxx>]

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 <serg@xxxxxxxxxxx> 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
>
>
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