Optimize query through the order of columns in index - mysql

Question

I had a table that is holding a domain and id
the query is
select distinct domain
from user
where id = '1'
the index is using the order idx_domain_id is faster than idx_id_domain
if the order of the execution is
(FROM clause,WHERE clause,GROUP BY clause,HAVING clause,SELECT
clause,ORDER BY clause)
then the query should be faster if it use the sorted where columns than the select one.
at 15:00 to 17:00 it show the same query i am working on
https://serversforhackers.com/laravel-perf/mysql-indexing-three
the table has a 4.6 million row.
time using idx_domain_id
time after change the order

Answer 1

This is your query:
select distinct first_name
from user
where id = '1';
You are observing that user(first_name, id) is faster than user(id, firstname).
Why might this be the case? First, this could simply be an artifact of how your are doing the timing. If your table is really small (i.e. the data fits on a single data page), then indexes are generally not very useful for improving performance.
Second, if you are only running the queries once, then the first time you run the query, you might have a "cold cache". The second time, the data is already stored in memory, so it runs faster.
Other issues can come up as well. You don't specify what the timings are. Small differences can be due to noise and might be meaningless.
You don't provide enough information to give a more definitive explanation. That would include:
Repeated timings run on cold caches.
Size information on the table and the number of matching rows.
Layout information, particularly the type of id.
Explain plans for the two queries.

Answer 2

select distinct domain
from user
where id = '1'
Since id is the PRIMARY KEY, there is at most one row involved. Hence, the keyword DISTINCT is useless.
And the most useful index is what you already have, PRIMARY KEY(id). It will drill down the BTree to find id='1' and deliver the value of domain that is sitting right there.
On the other hand, consider
select distinct domain
from user
where something_else = '1'
Now, the obvious index is INDEX(something_else, domain). This is optimal for the WHERE clause, and it is "covering" (meaning that all the columns needed by the query exist in the index). Swapping the columns in the index will be slower. Meanwhile, since there could be multiple rows, DISTINCT means something. However, it is not the logical thing to use.
Concerning your title question (order of columns): The = columns in the WHERE clause should come first. (More details in the link below.)
DISTINCT means to gather all the rows, then de-duplicate them. Why go to that much effort when this gives the same answer:
select domain
from user
where something_else = '1'
LIMIT 1
This hits only one row, not all the 1s.
Read my Indexing Cookbook.
(And, yes, Gordon has a lot of good points.)