What is the default select order in PostgreSQL or MySQL?

问题

I have read in the PostgreSQL docs that without an ORDER statement, SELECT will return records in an unspecified order.

Recently on an interview, I was asked how to SELECT records in the order that they inserted without an PK or created_at or other field that can be used for order. The senior dev who interviewed me was insistent that without an ORDER statement the records will be returned in the order that they were inserted.

Is this true for PostgreSQL? Is it true for MySQL? Or any other RDBMS?

回答1:

I can answer for MySQL. I don't know for PostgreSQL.

The default order is not the order of insertion, generally.

In the case of InnoDB, the default order depends on the order of the index read for the query. You can get this information from the EXPLAIN plan.

For MyISAM, it returns orders in the order they are read from the table. This might be the order of insertion, but MyISAM will reuse gaps after you delete records, so newer rows may be stored earlier.

None of this is guaranteed; it's just a side effect of the current implementation. MySQL could change the implementation in the next version, making the default order of result sets different, without violating any documented behavior.

So if you need the results in a specific order, you should use ORDER BY on your queries.

回答2:

Following BK's answer, and by way of example...

DROP TABLE IF EXISTS my_table;

CREATE TABLE my_table(id INT NOT NULL) ENGINE = MYISAM;

INSERT INTO my_table VALUES (1),(9),(5),(8),(7),(3),(2),(6);

DELETE FROM my_table WHERE id = 8;

INSERT INTO my_table VALUES (4),(8);

SELECT * FROM my_table;
+----+
| id |
+----+
|  1 |
|  9 |
|  5 |
|  4 | -- is this what
|  7 |
|  3 |
|  2 |
|  6 |
|  8 | -- we expect?
+----+

回答3:

Without an ORDER BY clause, the database is free to return rows in any order. There is no guarantee that rows will be returned in the order they were inserted.

With MySQL (InnoDB), we observe that rows are typically returned in the order by an index used in the execution plan, or by the cluster key of a table.

It is not difficult to craft an example...

CREATE TABLE foo 
( id INT NOT NULL
, val VARCHAR(10) NOT NULL DEFAULT ''
, UNIQUE KEY (id,val) 
) ENGINE=InnoDB; 

INSERT INTO foo (id, val) VALUES (7,'seven') ;
INSERT INTO foo (id, val) VALUES (4,'four') ; 

SELECT id, val FROM foo ;

MySQL is free to return rows in any order, but in this case, we would typically observe that MySQL will access rows through the InnoDB cluster key.

  id   val
----   ----- 
   4   four
   7   seven

Not at all clear what point the interviewer was trying to make. If the interviewer is trying to sell the idea, given a requirement to return rows from a table in the order the rows were inserted, a query without an ORDER BY clause is ever the right solution, I'm not buying it.

We can craft examples where rows are returned in the order they were inserted, but that is a byproduct of the implementation, ... not guaranteed behavior, and we should never rely on that behavior to satisfy a specification.

来源：https://stackoverflow.com/questions/60582206/what-is-the-default-select-order-in-postgresql-or-mysql

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mysql

postgresql