Are nested queries faster?

A general rule is that joins are faster in most cases (99\%). The more data tables have, the subqueries are slower. The less data tables have, the subqueries have equivalent speed as joins. The subqueries are simpler, easier to understand, and easier to read.

How can I speed up MySQL query execution?

MySQL has a built-in slow query log. To use it, open the my. cnf file and set the slow_query_log variable to “On.” Set long_query_time to the number of seconds that a query should take to be considered slow, say 0.2. Set slow_query_log_file to the path where you want to save the file.

Can you have multiple SELECT statements?

To combine two or more SELECT statements to form a single result table, use the set operators: UNION, EXCEPT or INTERSECT.

Are nested queries typically more efficient than join queries?

Conclusion : A subquery is easier to write, but a joint might be better optimized by the server. For example a Left Outer join typically works faster because servers optimize it.

What is the difference between select distinct and group by?

Distinct is used to find unique/distinct records where as a group by is used to group a selected set of rows into summary rows by one or more columns or an expression. The group by gives the same result as of distinct when no aggregate function is present.

How optimize MySQL query with multiple joins?

2 Answers

1. Check if you really have to select every column in all of the tables?
2. You may also want to consider reducing the load on the database by using caching applications like sphinxsearch and memcached.
3. Check none of your joins are to views rather than actual tables.

How do I join multiple select statements in MySQL?

The MySQL UNION operator is used to combine the result sets of 2 or more SELECT statements. It removes duplicate rows between the various SELECT statements. Each SELECT statement within the UNION operator must have the same number of fields in the result sets with similar data types.

How can I improve my multiple joins performance?

You should see the Query Execution Plan and optimize the query section by section. The overall optimization process consists of two main steps: Isolate long-running queries….

1. Yakshemash, tomtom.
2. Because they are expensivbe.
3. Distinct checks all rows in the returning data.
4. I agree about the use of DISTINCT .

Do nested subqueries work in MySQL?

Some of my readers may be aware that nested subqueries such as “SELECT * FROM widgets WHERE id IN (SELECT …)”, don’t work all that well in MYSQL. While the syntax is usually correct, the performance issues in practice can be horrendous.

What is the problem with nested queries?

The problem with nested queries is that in many circumstances they will perform just fine, but change the data slightly and they can seriously harm database performance in MySQL. For example, strange things can happen if the subquery returns no records so that you end up with “WHERE id IN ()”.

Are query optimizers smart enough to optimize nested queries?

Most query optimizers are not smart enough to do this since finding such a conversion would take too long, or in computing terms would require too large a search space and near-infinite time. In fact, many query optimizers will flat out refuse to optimize nested queries if it sees them.

Why are repeated queries faster with InnoDB?

With efficient use of the InnoDB buffer pool , MyISAM key cache, and the MySQL query cache, repeated queries run faster because the results are retrieved from memory the second and subsequent times.