Query time suddenly increased - mysql

Question

I have MariaDB 10.1.14, For a long time I'm doing the following query without problems (it tooks about 3 seconds):
SELECT
sum(transaction_total) as sum_total,
count(*) as count_all,
transaction_currency
FROM
transactions
WHERE
DATE(transactions.created_at) = DATE(CURRENT_DATE)
AND transaction_type = 1
AND transaction_status = 2
GROUP BY
transaction_currency
Suddenly, I'm not sure exactly why, this query take about 13 seconds.
This is the EXPLAIN:
And those are the all indexes of transactions table:
What is the reason for the sudden query time increase? and how can I decrease it?

Answer 1

If you are adding more data to your table the query time will increase.
But you can do a few things to improve the performance.
Create a composite index for ( transaction_type, transaction_status, created_at)
Remove the DATE() functions (or any function) from your fields, because that doesn't allow engine use the index. CURRENT_DATE is a constant so there doesn't matter, but isn't necessary because already return DATE
if created_at isnt date you can use
created_at >= CURRENT_DATE and created_at < CURRENT_DATE + 1
or create a different field to only save the date part.

Answer 2

+1 to answer from #JuanCarlosOropeza, but you can go a little further with the index.
ALTER TABLE transactions ADD INDEX (
transaction_type,
transaction_status,
created_at,
transaction_currency,
transaction_total
);
As #RickJames mentioned in comments, the order of columns is important.
First, columns in equality comparisons
Next, you can index one column that is used for a range comparison (which is anything besides equality), or GROUP BY or ORDER BY. You have both range comparison and GROUP BY, but you can only get the index to help with one of these.
Last, other columns needed for the query, if you think you can get a covering index.
I describe more detail about index design in my presentation How to Design Indexes, Really (video: https://www.youtube.com/watch?v=ELR7-RdU9XU).
You're probably stuck with the "using temporary" since you have a range condition and also a GROUP BY referencing different columns. But you can at least eliminate the "using filesort" by this trick:
...
GROUP BY
transaction_currency
ORDER BY NULL
Supposing that it's not important to you which order the rows of the query results return in.

Answer 3

I don't know what has made your query slower. More data? Fragmentation? New DB version?
However, I am surprised to see that there is no index really supporting the query. You should have a compound index starting with the column with highest cardinality (the date? well, you can try different column orders and see which index the DBMS picks for the query).
create index idx1 on transactions(created_at, transaction_type, transaction_status);
If created_at contains a date part, then you may want to create a computed column created_on only containing the date and index that instead.
You can even extend this index to a covering index (where clause fields followed by group by clause fields followed by select clause fields):
create index idx2 on transactions(created_at, transaction_type, transaction_status,
transaction_currency, transaction_total);