MySQL GROUP BY with Using Temporary unnecessarily? - mysql

Question

I am trying to optimize a query. Using EXPLAIN tells me it is Using temporary. This is really inefficient given the size of the table (20m+ records). Looking at the MySQL documentation Internal Temporary Tables I don't see anything that would imply the need for a Temporary table in my query. I also tried setting the ORDER BY to the same as the GROUP BY, but still says Using Temporary and query takes forever to run. I am using MySQL 5.7.
Is there a way to avoid using a temporary table for this query:
SELECT url,count(*) as sum
FROM `digital_pageviews` as `dp`
WHERE `publisher_uuid` = '8b83120e-3e19-4c34-8556-7b710bd7b812'
GROUP BY url
ORDER BY NULL;
This is my table schema:
create table digital_pageviews
(
id int unsigned auto_increment
primary key,
visitor_uuid char(36) null,
publisher_uuid char(36) default '' not null,
property_uuid char(36) null,
ip_address char(15) not null,
referrer text null,
url_delete text null,
url varchar(255) null,
url_tmp varchar(255) null,
meta text null,
date_created timestamp not null,
date_updated timestamp null
)
collate = utf8_unicode_ci;
create index digital_pageviews_url_index
on digital_pageviews (url);
create index ndx_date_created
on digital_pageviews (date_created);
create index ndx_property_uuid
on digital_pageviews (property_uuid);
create index ndx_publisher_uuid
on digital_pageviews (publisher_uuid);
create index ndx_visitor_uuid_page
on digital_pageviews (visitor_uuid);

Answer 1

The reason it needs a temporary table is that it cannot both filter by publisher_uuid and sort on a column without an index to do both. The first step is to filter by publisher_uuid, so it uses the index on publisher_uuid.
However, next it has to group by and order the records, which will require a temporary table because it cannot use an index which will do this. The reason it cannot use an index is that it already used the publisher_uuid, which is not indexed on the url field to do the group by or on the field you are ordering by.
To filter where publisher_uuid = '8b83120e-3e19-4c34-8556-7b710bd7b812', group by url, and order by url, create an index with these fields in this order:
publisher_uuid
url
create index ndx_publisher_uuid
on digital_pageviews (publisher_uuid, url);