maria-discuss team mailing list archive

[Sergei Golubchik <serg@xxxxxxxxxxx>]

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