Default row ordering for select query in oracle

问题:

In Oracle, what is the the default ordering of rows for a select query if no \"order by\" clause is specified.

Is it

1. the order in which the rows were inserted
2. there is no default ordering at all
3. none of the above.

回答1:

According to Tom Kyte: \"Unless and until you add \"order by\" to a query, you cannot say ANYTHING about the order of the rows returned. Well, short of \'you cannot rely on the order of the rows being returned\'.\"

See this question at asktom.com.

As for ROWNUM, it doesn\'t physically exist, so it can\'t be \"freed\". ROWNUM is assigned after a record is retrieved from a table, which is why \"WHERE ROWNUM = 5\" will always fail to select any records.

@ammoQ: you might want to read this AskTom article on GROUP BY ordering. In short:

Does a Group By clause in an Query gaurantee that the output data will be sorted on the Group By columns in order, even if there is NO Order By clause?

and we said...

ABSOLUTELY NOT,

It never has, it never did, it never will.

回答2:

There is no explicit default ordering. For obvious reasons, if you create a new table, insert a few rows and do a \"select *\" without a \"where\" clause, it will (very likely) return the rows in the order they were inserted.

But you should never ever rely on a default order happening. If you need a specific order, use an \"order by\" clause. For example, in Oracle versions up to 9i, doing a \"group by\" also caused the rows to be sorted by the group expression. In 10g, this behaviour does no longer exist! Upgrading Oracle installations has caused me some work because of this.

回答3:

It has already been said that Oracle is allowed to give you the rows in any order it wants, when you don\'t specify an ORDER BY clause. Speculating what the order will be when you don\'t specify the ORDER BY clause is pointless. And relying on it in your code, is a \"career limiting move\".

A simple example:

SQL> create table t as select level id from dual connect by level <= 10
  2  /

Tabel is aangemaakt.

SQL> select id from t
  2  /

        ID
----------
         1
         2
         3
         4
         5
         6
         7
         8
         9
        10

10 rijen zijn geselecteerd.

SQL> delete t where id = 6
  2  /

1 rij is verwijderd.

SQL> insert into t values (6)
  2  /

1 rij is aangemaakt.

SQL> select id from t
  2  /

        ID
----------
         1
         2
         3
         4
         5
         7
         8
         9
        10
         6

10 rijen zijn geselecteerd.

And this is only after a simple delete+insert. And there are numerous other situations thinkable. Parallel execution, partitions, index organised tables to name just a few.

Bottom line, as already very well said by ammoQ: if you need the rows sorted, use an ORDER BY clause.

回答4:

You absolutely, positively cannot rely on any ordering unless you specify order by. For Oracle in particular, I\'ve actually seen the exact same query (without joins), run twice within a few seconds of each other, on a table that didn\'t change in the interim, return a wildly different order. This seems to be more likely when the result set is large.

The parallel execution mentioned by Rob van Wijk probably explains this. See also Oracle\'s Using Parallel Execution doc.

回答5:

You can modify the order in which data is stored into the table by INSERT with the ORGANIZATION clause of the CREATE TABLE statement

回答6:

It is impacted by index , if there is index ,it will return a ascending order , if there is not any index ,it will return the order inserted .

回答7:

Although, it should be rownnum (your #2), it really isn\'t guaranteed and you shouldn\'t trust it 100%.

回答8:

I believe it uses Oracle\'s hidden Rownum attribute.

So your #1 is probably right assuming there were no deletes done that might have freed rownums for later use.

EDIT: As others have said, you really shouldn\'t rely on this, ever. Besides deletes theres a lot of different conditions that can affect the default sorting behavior.