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@mariadb.org> 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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