Which is fastest? SELECT SQL_CALC_FOUND_ROWS FROM `table`, or SELECT COUNT(*)

问题:

When you limit the number of rows to be returned by a SQL query, usually used in paging, there are two methods to determine the total number of records:

Method 1

Include the SQL_CALC_FOUND_ROWS option in the original SELECT, and then get the total number of rows by running SELECT FOUND_ROWS():

SELECT SQL_CALC_FOUND_ROWS * FROM table WHERE id > 100 LIMIT 10; SELECT FOUND_ROWS();   

Method 2

Run the query normally, and then get the total number of rows by running SELECT COUNT(*)

SELECT * FROM table WHERE id > 100 LIMIT 10; SELECT COUNT(*) FROM table WHERE id > 100;   

Which method is the best / fastest?

回答1:

It depends. See the MySQL Performance Blog post on this subject: http://www.mysqlperformanceblog.com/2007/08/28/to-sql_calc_found_rows-or-not-to-sql_calc_found_rows/

Just a quick summary: Peter says that it depends on your indexes and other factors. Many of the comments to the post seem to say that SQL_CALC_FOUND_ROWS is almost always slower - sometimes up to 10x slower - than running two queries.

回答2:

When choosing the "best" approach, a more important consideration than speed might be the maintainability and correctness of your code. If so, SQL_CALC_FOUND_ROWS is preferable because you only need to maintain a single query. Using a single query completely precludes the possibility of a subtle difference between the main and count queries, which may lead to an inaccurate COUNT.

回答3:

According to the following article: https://www.percona.com/blog/2007/08/28/to-sql_calc_found_rows-or-not-to-sql_calc_found_rows/

If you have an INDEX on your where clause (if id is indexed in your case), then it is better not to use SQL_CALC_FOUND_ROWS and use 2 queries instead, but if you don't have an index on what you put in your where clause (id in your case) then using SQL_CALC_FOUND_ROWS is more efficient.

回答4:

IMHO, the reason why 2 queries

SELECT * FROM count_test WHERE b = 666 ORDER BY c LIMIT 5; SELECT count(*) FROM count_test WHERE b = 666; 

are faster than using SQL_CALC_FOUND_ROWS

SELECT SQL_CALC_FOUND_ROWS * FROM count_test WHERE b = 555 ORDER BY c LIMIT 5; 

has to be seen as a particular case.

It in facts depends on the selectivity of the WHERE clause compared to the selectivity of the implicit one equivalent to the ORDER + LIMIT.

As Arvids told in comment (http://www.mysqlperformanceblog.com/2007/08/28/to-sql_calc_found_rows-or-not-to-sql_calc_found_rows/#comment-1174394), the fact that the EXPLAIN use, or not, a temporay table, should be a good base for knowing if SCFR will be faster or not.

But, as I added (http://www.mysqlperformanceblog.com/2007/08/28/to-sql_calc_found_rows-or-not-to-sql_calc_found_rows/#comment-8166482), the result really, really depends on the case. For a particular paginator, you could get to the conclusion that “for the 3 first pages, use 2 queries; for the following pages, use a SCFR” !

回答5:

Removing some unnecessary SQL and then COUNT(*) will be faster than SQL_CALC_FOUND_ROWS. Example:

• SELECT Person.Id, Person.Name, Job.Description, Card.Number
• FROM Person
• JOIN Job ON Job.Id = Person.Job_Id
• LEFT JOIN Card ON Card.Person_Id = Person.Id
• WHERE Job.Name = 'WEB Developer'
• ORDER BY Person.Name

Then count without unnecessary part:

• SELECT COUNT(*)
• FROM Person
• JOIN Job ON Job.Id = Person.Job_Id
• LEFT JOIN Card ON Card.Person_Id = Person.Id
• WHERE Job.Name = 'WEB Developer'
• ORDER BY Person.Name

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