How to optimize query with many joins? - mysql

Question

I have simple but long query which count the content of the result it takes about 14 seconds. the count itself on the main table takes less than a second but after multiple join the delay is too high as follow
Select Count(Distinct visits.id) As Count_id
From visits
Left Join clients_locations ON visits.client_location_id = clients_locations.id
Left Join clients ON clients_locations.client_id = clients.id
Left Join locations ON clients_locations.location_id = locations.id
Left Join users ON visits.user_id = users.id
Left Join potentialities ON clients_locations.potentiality = potentialities.id
Left Join classes ON clients_locations.class = classes.id
Left Join professions ON clients.profession_id = professions.id
Inner Join specialties ON clients.specialty_id = specialties.id
Left Join districts ON locations.district_id = districts.id
Left Join provinces ON districts.province_id = provinces.id
Left Join locations_types ON locations.location_type_id = locations_types.id
Left Join areas ON clients_locations.area_id = areas.id
Left Join calls ON calls.visit_id = visits.id
The output of explain is
+---+---+---+---+---+---+---+---+---+---+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+---+---+---+---+---+---+---+---+---+---+
| 1 | SIMPLE | specialties | index | PRIMARY | specialty_name | 52 | NULL | 53 | Using index |
| 1 | SIMPLE | clients | ref | PRIMARY,specialty | specialty | 4 | crm_db.specialties.id | 143 | |
| 1 | SIMPLE | clients_locations | ref | PRIMARY,client_id | client_id | 4 | crm_db.clients.id | 1 | |
| 1 | SIMPLE | locations | eq_ref | PRIMARY | PRIMARY | 4 | crm_db.clients_locations.location_id | 1 | |
| 1 | SIMPLE | districts | eq_ref | PRIMARY | PRIMARY | 4 | crm_db.locations.district_id | 1 | Using where |
| 1 | SIMPLE | visits | ref | unique_visit,client_location_id | unique_visit | 4 | crm_db.clients_locations.id | 4 | Using index |
| 1 | SIMPLE | calls | ref | call_unique,visit_id | call_unique | 4 | crm_db.visits.id | 1 | Using index |
+---+---+---+---+---+---+---+---+---+---+
Update 1
The above query used with dynamic where statement $sql = $sql . "Where ". $whereFilter but the i submitted it in simple form . So do not consider the answer just eleminate the joins :)
Update 2
Here is example of dynamic filtering
$temp = $this->province_id;
if ($temp != null) {
$whereFilter = $whereFilter . " and provinces.id In ($temp) ";
}
But in startup case which is our case no where statement

Answer 1

Left joins always return a row from the first table, but may return multiple rows if there are multiple matching rows. But because you are counting distinct visit rows, left joining to another table while counting distinct visits is the same as just counting the rows of visits. Thus the only joins that affect the result are inner joins, so you can remove all "completely" left joined tables without affecting the result.
What I mean by "completely" is that some left joined tables are effectively inner joined; the inner join to specialty requires the join to clients to succeed and thus also be an inner join, which in turn requires the join to clients_locations to succeed and thus also be an inner join.
Your query (as posted) can be reduced to:
Select Count(Distinct visits.id) As Count_id
From visits
Join clients_locations ON visits.client_location_id = clients_locations.id
Join clients ON clients_locations.client_id = clients.id
Join specialties ON clients.specialty_id = specialties.id
Removing all those unnecessary joins will however greatly improve the runtime of your query, not only because there are less joins to make but also because the resulting rowset size could be enormous when you consider that the size is the product of the matches in all the tables (not the sum.
For maximum performance, create a covering indexes on all id-and-fk columns:
create index visits_id_client_location_id on visits(id, client_location_id);
create index clients_locations_id_client_id on clients_locations(id, client_id);
create index clients_id_specialty_id on clients(id, specialty_id);
so index-only scans can be used where possible. I assume there are indexes on the PK columns.

Answer 2

You don't seem to have any (or much) intentional filtering. If you want to know the number of visits referred to in calls, I would propose:
select count(distinct c.visit_id)
from calls c;

Answer 3

in order to optimize the whole process you can dynamically construct the pre-where SQL according to the filters you are going to apply. Like:
// base select and left join
$preSQL = "Select Count(Distinct visits.id) As Count_id From visits ";
$preSQL .= "Left Join clients_locations ON visits.client_location_id = clients_locations.id ";
// filtering by province_id
$temp = $this->province_id;
if ($temp != null) {
$preSQL .= "Left Join locations ON clients_locations.location_id = locations.id ";
$preSQL .= "Left Join districts ON locations.district_id = districts.id ";
$preSQL .= "Left Join provinces ON districts.province_id = provinces.id ";
$whereFilter = "provinces.id In ($temp) ";
}
$sql = $preSQL . "Where ". $whereFilter;
// ...
If you are using multiple filters you can put all inner/left-join strings in an array and then after analysing the request, you can construct your $preSQL using the minimum of joins.

Answer 4

Use COUNT(CASE WHEN visit_id!="" THEN 1 END) as visit.
Hope this will help

Answer 5

Isn't it just:
SELECT COUNT(id)
FROM visits
because all the left outer joins also return a visits.id when theres no matching clients, ..., calls and id's ought to be unique?
Different hint: The one inner join also is only effective when a client exists. Generally when needing inner joins they must be put as high/near as possible to the source table, so in your example it would have been best in the line after "left join clients".

Answer 6

I didn't understand too much your idea, specially your INNER JOIN that will tranform some LEFT in INNER JOINs, it seems strange, but lets try a solution:
Usually the LEFT JOINs has a very bad performance, and I think you'll need them only if you'll use them in WHERE clause, then you can include them with INNER JOIN only if you'll use them.
For example:
$query = "Select Count(Distinct visits.id) As Count_id From visits ";
if($temp != null){
$query .= " INNER JOIN clients_locations ON visits.client_location_id = clients_locations.id ";
$query .= " INNER JOIN locations ON clients_locations.location_id = locations.id ";
$query .= " INNER JOIN locations ON clients_locations.location_id = locations.id ";
$query .= " INNER JOIN districts ON locations.district_id = districts.id "
$query .= " INNER JOIN provinces ON districts.province_id = provinces.id ";
$whereFilter .= " and provinces.id In ($temp) ";
}
I think it'll help your performance and it'll works as you need.