Re: index is not used why?

[Thomas Jüntgen]

Thanks for your answer!
The original query scan 15710 rows of 15710 rows.

Your first solution is doing the following

scan 15710 rows of 15710 (type =ALL)
scan 11783 rows of 15710 (type =ALL)
scan 198 rows of 15710 (type=ref)


Your second solution
scan 15710 rows of 15710 (type =index, used key=IDX_RESOURCE_GROUP)
scan 1 rows of 15710 (type =ref, used key=PRIMARY)

Regarding your question of ROW_FORMAT=FIXED. I read that this format could be useful for performance.

Anyway both solution are not faster then the first query.
Also this Query
select c_id
FROM t_event_calendar a
WHERE (a.n_resource_id = 4
OR a.n_group_id IN (1)
OR ( a.n_group_id IS NULL
AND a.n_resource_id IS NULL))
scan 15710 rows of 15710 (type =index, used key=IDX_RESOURCE_GROUP)
result 15224 rows
Why does MySQL think to scan all rows instead using the key?

Thanks for your help
Thomas