Explain SQL and Query optimization

Explain SQL (in phpmyadmin) of a query that is taking more than 5 seconds is giving me the above. I read that we can study the Explain SQL to optimize a query. Can anyone tell if this Explain SQL telling anything as such?

Thanks guys.

Edit:

The query itself:

SELECT 
a.`depart` , a.user, 
m.civ, m.prenom, 开发者_开发百科m.nom, 
CAST( GROUP_CONCAT( DISTINCT concat( c.id, '~', c.prenom, ' ', c.nom ) ) AS char ) AS coordinateur, 
z.dr
FROM `0_activite` AS a
JOIN `0_member` AS m ON a.user = m.id
LEFT JOIN `0_depart` AS d ON ( m.depart = d.depart AND d.rank = 'mod' AND d.user_sec =2 )
LEFT JOIN `0_member` AS c ON d.user_id = c.id
LEFT JOIN `zone_base` AS z ON m.depart = z.deprt_num
GROUP BY a.user

Edit 2:

Structures of the two tables a and d

Edit 3:

What I want in this query?

I first want to get the value of 'depart' and 'user' (which is an id) from the table 0_activite. Next, I want to get name of the person (civ, prenom and name) from 0_member whose id I am getting from 0_activite via 'user', by matching 0_activite.user with 0_member.id. Here depart is short of department which is also an id.

So at this point, I have depart, id, civ, nom and prenom of a person from two tables, 0_activite and 0_member.

Next, I want to know which dr is related with this depart, and this I get from zone_base. The value of depart is same in both 0_activite and 0_member.

Then comes the trickier part. A person from 0_member can be associated with multiple departs and this is stored in 0_depart. Also, every user has a level, one of what is 'mod', stands for moderator. Now I want to get all the people who are moderators in the depart from where the first user is, and then get those moderaor's name from 0_member again. I also have a variable user_sec, but this is probably less important in this context, though I cannot overlook it.

This is what makes the query a tricky one. 0_member is storing id, name of users, + one depart, 0_depart is storing all departs of users, one line for each depart, and 0_activite is storing some other stuffs and I want to relate those through userid of 0_activite and the rest.

Hope I have been clear. If I am not, please let me know and I will try again to edit this post.

Many many thanks again.

Aside from the few answers provided by the others here, it might help to better understand the "what do I want" from the query. As you've accepted a rather recent answer from me in another of your questions, you have filters applied by department information.

Your query is doing a LEFT join at the Department table by rank = 'mod' and user_sec = 2. Is your overall intent to show ALL records in the 0_activite table REGARDLESS of a valid join to the 0_Depart table... and if there IS a match to the 0_Depart table, you only care about the 'mod' and 2 values?

If you only care about those people specifically associated with the 0_depart with 'mod' and 2 conditions, I would reverse the query starting with THIS table first, then join to the rest.

Having keys on tables via relationship or criteria is always a performance benefit (vs not having the indexes).

Start your query with whatever would be your smallest set FIRST, then join to other tables.

From clarification in your question... I would start with the inner-most... Who it is and what departments are they associated with... THEN get the moderators (from department where condition)... Then get actual moderator's name info... and finally out to your zone_base for the dr based on the department of the MODERATOR...

select STRAIGHT_JOIN
      DeptPerMember.*
      Moderator.Civ as ModCiv,
      Moderator.Prenom as ModPrenom,
      Moderator.Nom as ModNom,
      z.dr
   from 
      ( select
              m.ID,
              m.Depart,
              m.Civ,
              m.Prenom,
              m.Nom
           from
              0_Activite as a
                 join 0_member m
                    on a.User = m.ID
                    join 0_Depart as d
                       on m.depart = d.depart  ) DeptPerMember

      join 0_Depart as DeptForMod
         on DeptPerMember.Depart = DeptForMod.Depart
         and DeptForMod.rank = 'mod'
         and DeptForMod.user_sec = 2 

         join 0_Member as Moderator
            on DeptForMod.user_id = Moderator.ID

            join zone_base z
               on Moderator.depart = z.deprt_num

Notice how I tier'd the query to get each part and joined to the next and next and next. I'm building the chain based on the results of the previous with clear "alias" references for clarification of content. Now, you can get whatever respective elements from any of the levels via their distinct "alias" references...

The output from EXPLAIN is showing us that the first and third tables listed (a & d) are not having any indexes utilised by the database engine in executing this query. The key column is NULL for both - which is a shame since both are 'large' tables (OK, they're not really large, but compared to the rest of the tables they're the big 'uns).

Judging from the query, an index on user on 0_activite and an index on (depart, rank, user_sec) on 0_depart would go some way to improving performance.

you can see that columns key and key_len are null this means its not using any key in the possible_keys column. So table a and d are both scanning all rows. (check larger numbers in rows column. you want this smaller).

To deal with 0_depart: Make sure you have a key on (d.depart, d.rank,d.user_sec) which are part of the join of 0_depart.

To deal with 0_activite: I'm not positive but a GROUP column should be indexed too so you need a key on a.user