What is the “Default order by” for a mysql Innodb query that omits the Order by clause?

问题

So i understand and found posts that indicates that it is not recommended to omit the order by clause in a SQL query when you are retrieving data from the DBMS.

Resources & Post consulted (will be updated):

SQL Server UNION - What is the default ORDER BY Behaviour

When no 'Order by' is specified, what order does a query choose for your record set?

https://dba.stackexchange.com/questions/6051/what-is-the-default-order-of-records-for-a-select-statement-in-mysql

Questions :

See logic of the question below if you want to know more.

My question is : under mysql with innoDB engine, does anyone know how the DBMS effectively gives us the results ?

I read that it is implementation dependent, ok, but is there a way to know it for my current implementation ?

Where is this defined exactly ?

Is it from MySQL, InnoDB , OS-Dependent ?

Isn't there some kind of list out there ?

Most importantly, if i omit the order by clause and get my result, i can't be sure that this code will still work with newer database versions and that the DBMS will never give me the same result, can i ?

Use case & Logic :

I'm currently writing a CRUD API, and i have table in my DB that doesn't contain an "id" field (there is a PK though), and so when i'm showing the results of that table without any research criteria, i don't really have a clue on what i should use to order the results. I mean, i could use the PK or any field that is never null, but it wouldn't make it relevant. So i was wondering, as my CRUD is supposed to work for any table and i don't want to solve this problem by adding an exception for this specific table, i could also simply omit the order by clause.

Final Note :

As i'm reading other posts, examples and code samples, i'm feeling like i want to go too far. I understand that it is common knowledge that it's just a bad practice to omit the Order By clause in a request and that there is no reliable default order clause, not to say that there is no order at all unless you specify it.

I'd just love to know where this is defined, and would love to learn how this works internally or at least where it's defined (DBMS / Storage Engine / OS-Dependant / Other / Multiple criteria). I think it would also benefit other people to know it, and to understand the inners mechanisms in place here.

Thanks for taking the time to read anyway ! Have a nice day.

回答1:

Without a clear ORDER BY, current versions of InnoDB return rows in the order of the index it reads from. Which index varies, but it always reads from some index. Even reading from the "table" is really an index—it's the primary key index.

As in the comments above, there's no guarantee this will remain the same in the next version of InnoDB. You should treat it as a coincidental behavior, it is not documented and the makers of MySQL don't promise not to change it.

Even if their implementation doesn't change, reading in index order can cause some strange effects that you might not expect, and which won't give you query result sets that makes sense to you.

For example, the default index is the clustered index, PRIMARY. It means index order is the same as the order of values in the primary key (not the order in which you insert them).

mysql> create table mytable ( id int primary key, name varchar(20));

mysql> insert into mytable values (3, 'Hermione'), (2, 'Ron'), (1, 'Harry');

mysql> select * from mytable;
+----+----------+
| id | name     |
+----+----------+
|  1 | Harry    |
|  2 | Ron      |
|  3 | Hermione |
+----+----------+

But if your query uses another index to read the table, like if you only access column(s) of a secondary index, you'll get rows in that order:

mysql> alter table mytable add key (name);

mysql> select name from mytable;
+----------+
| name     |
+----------+
| Harry    |
| Hermione |
| Ron      |
+----------+

This shows it's reading the table by using an index-scan of that secondary index on name:

mysql> explain select name from mytable;
+----+-------------+---------+-------+---------------+------+---------+------+------+-------------+
| id | select_type | table   | type  | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+---------+-------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | mytable | index | NULL          | name | 83      | NULL |    3 | Using index |
+----+-------------+---------+-------+---------------+------+---------+------+------+-------------+

In a more complex query, it can become very tricky to predict which index InnoDB will use for a given query. The choice can even change from day to day, as your data changes.

All this goes to show: You should just use ORDER BY if you care about the order of your query result set!

回答2:

Bill's answer is good. But not complete.

• If the query is a UNION, it will (I think) deliver first the results of the first SELECT (according to the rules), then the results of the second. Also, if the table is PARTITIONed, it is likely to do a similar thing.

• GROUP BY may sort by the grouping expressions, thereby leading to a predictable order, or it may use a hashing technique, which scrambles the rows. I don't know how to predict which.

• A derived table used to be an ordered list that propagates into the parent query's ordering. But recently, the ORDER BY is being thrown away in that subquery! (Unless there is a LIMIT.)

Bottom Line: If you care about the order, add an ORDER BY, even if it seems unnecessary based on this Q & A.

MyISAM, in contrast, starts with this premise: The default order is the order in the .MYD file. But DELETEs leave gaps, UPDATEs mess with the gaps, and INSERTs prefer to fill in gaps over appending to the file. So, the row order is rather unpredictable. ALTER TABLE x ORDER BY y temporarily sets the .MYD order; this 'feature' does not work for InnoDB.

来源：https://stackoverflow.com/questions/49815703/what-is-the-default-order-by-for-a-mysql-innodb-query-that-omits-the-order-by