Inner join in mysql take a long time - mysql

Question

I have table contacts with more than 1,000,000 and other table cities which have about 20,000 records. Need to fetch all cities which have used in contacts table.
Contacts table have following columns
Id, name, phone, email, city, state, country, postal, address, manager_Id
cities table have
Id, city
I used Inner join for this, but its taking a long time to go. Query takes more than 2 minutes to execute.
I used this query
SELECT cities.* FROM cities
INNER JOIN contacts ON contacts.City = cities.city
WHERE contacts.manager_Id= 1
created index on manager_Id as well. But still its very slow.

Answer 1

for better performance you could add index
on table cities column city
on table contacts a composite index on columns (manager_id, city)

Answer 2

Filter contacts first and then join to cities:
SELECT ct.*
FROM cities ct INNER JOIN (
SELECT city FROM contacts
WHERE manager_Id = 1
) cn ON cn.city = ct.city
You need indexes for city in both tables and for manager_id in contacts.

Answer 3

As others have pointed out about having proper index, I am taking it a bit more for clarification. You are specifically looking for contacts where the MANAGER ID = 1. This is not expected to be one person, but could be many people. So having the MANAGER ID in the first position will optimize get me all people for that manager. By having the city as part of the index via (manager_id, city), you are pulling the two data elements you need to optimize as part of the index. This way the engine does not have to go to the raw data pages to get the other part of interest.
Now, From that, you want all the city information (hence the join to city table on that ID).
Since you are only querying the CITIES and not the actual contact people information, you probably want to have DISTINCT City ID. Lets say a manager is responsible for 50 people and most of them live in the same city or neighboring. You may have 5 distinct cities? That too will limit your result set of joining.
Having said that, I would do a follows, and with MySQL, using STRAIGHT_JOIN can help optimize by "do the query as I wrote it, don't think for me".
select STRAIGHT_JOIN
cty.*
from
( select distinct c.City
from Contacts c
where c.Manager_ID = 1 ) PQ
JOIN Cities cty
on PQ.City = Cty.City
The "PQ" is an alias representing my "pre-query" of just DISTINCT cities for a given manager.
Again, have one index on Contacts table on (manager_id, city). On the city table, I would expect and index on (city).

Answer 4

You need two indexes, one on each table.
On the contacts table, first index manager_Id, then City
CREATE INDEX idx_contacts_mgr_city ON contacts(manager_Id, City);
On the cities table, just index `City.

Answer 5

Is the 'City' field from the table 'Contacts' a VARCHAR?
If that's the case, I see multiple things here.
First of all, since you have already have the 'Id' for the corresponding city in your 'cities' tables, I don't see why not to use the same 'Id' from the 'cities' table for the 'Contacts' table.
You can add the 'IdCity' field to the 'Contacts' table so you don't have to modify your existing records.
You'll have to insert the 'IdCity' manually though for each of your records, or you can create a Query using 'cities' table and then compare the 'idCity' but insert the 'city' (city name) in your 'Contacts' table.
Returning to your query:
Then, use an INT JOIN instead of a VARCHAR JOIN. Since you have many records, this can show up an important significance in performance.

Answer 6

It looks like you need to add two indexes, one on cities.city and one on (contacts.manager_Id, contacts.city). That should speed things up significantly.