Thread: Getting ID of last-inserted row

Hi there.

I never found an adequate (simple and efficient) method for getting
the primary key ID of the just-inserted row, and usually used
transactions and "select last value, ordered by id"-type queries to
get the last id value, or other ugly logic.

That was until I found how SQLalchemy[1] handles it for PostgreSQL.
What they do is:

1) First, get the next value from the sequence, eg:

SELECT nextval('clients_id_seq');

2) Then, run an insert statement, where the retrieved value is
explicitly given, rather than automatically assigned, eg:

INSERT INTO clients (id, name) VALUES (12345, 'John Smith');

(Where 12345 is the id retrieved from the previous query).

I wanted to add this info to the wiki[2], but there doesn't seem to be
a way to sign up.

Anyway, I thought that other people might find this info useful.

David.

[1] http://www.sqlalchemy.org/
[2] http://wiki.postgresql.org/wiki/Main_Page

In response to David :
> Hi there.
>
> I never found an adequate (simple and efficient) method for getting
> the primary key ID of the just-inserted row, and usually used
> transactions and "select last value, ordered by id"-type queries to
> get the last id value, or other ugly logic.

use currval() instead, see
http://www.postgresql.org/docs/current/static/functions-sequence.html
--
Andreas Kretschmer
Kontakt:  Heynitz: 035242/47150,   D1: 0160/7141639 (mehr: -> Header)
GnuPG-ID:   0x3FFF606C, privat 0x7F4584DA   http://wwwkeys.de.pgp.net

On 20/05/2009 11:17, A. Kretschmer wrote:
> In response to David :
>> Hi there.
>>
>> I never found an adequate (simple and efficient) method for getting
>> the primary key ID of the just-inserted row, and usually used
>> transactions and "select last value, ordered by id"-type queries to
>> get the last id value, or other ugly logic.
>
> use currval() instead, see
> http://www.postgresql.org/docs/current/static/functions-sequence.html

Also, you can do insert....returning... (as of version 8.2, I think):

INSERT INTO clients (id, name)
  VALUES (nextval('clients_id_seq'), 'John Smith')
  RETURNING id;

Ray.

------------------------------------------------------------------
Raymond O'Donnell, Director of Music, Galway Cathedral, Ireland
rod@iol.ie
Galway Cathedral Recitals: http://www.galwaycathedral.org/recitals
------------------------------------------------------------------

>> I never found an adequate (simple and efficient) method for getting
>> the primary key ID of the just-inserted row, and usually used
>> transactions and "select last value, ordered by id"-type queries to
>> get the last id value, or other ugly logic.
>
> use currval() instead, see
> http://www.postgresql.org/docs/current/static/functions-sequence.html
> --

I think I researched that before also, but I wasn't sure at the time
how safe it was against race conditions. Although I see now (reading
the docs again) that it is tracked for different sessions so it should
be safe. There might also be issues (for instance, where you are using
a complex database-handling library), where you can't always guarantee
that your currval() call is made at the right time. But most of the
time it should be fine. Thanks for the reminder.

> Also, you can do insert....returning... (as of version 8.2, I think):
>
> INSERT INTO clients (id, name)
>  VALUES (nextval('clients_id_seq'), 'John Smith')
>  RETURNING id;

Thanks. I think I saw that too, not too long ago, but forgot about it.
Some of the PostgreSQL services I use are on older versions, so I need
to use older syntax. But, this will be useful when the db version is
guaranteed to be recent. Thanks also for your reminder.

On Wed, May 20, 2009 at 4:35 AM, David <wizzardx@gmail.com> wrote:
>>> I never found an adequate (simple and efficient) method for getting
>>> the primary key ID of the just-inserted row, and usually used
>>> transactions and "select last value, ordered by id"-type queries to
>>> get the last id value, or other ugly logic.
>>
>> use currval() instead, see
>> http://www.postgresql.org/docs/current/static/functions-sequence.html
>> --
>
> I think I researched that before also, but I wasn't sure at the time
> how safe it was against race conditions. Although I see now (reading
> the docs again) that it is tracked for different sessions so it should
> be safe.

It is.

>> Also, you can do insert....returning... (as of version 8.2, I think):
>>
>> INSERT INTO clients (id, name)
>>  VALUES (nextval('clients_id_seq'), 'John Smith')
>>  RETURNING id;
>
> Thanks. I think I saw that too, not too long ago, but forgot about it.
> Some of the PostgreSQL services I use are on older versions, so I need
> to use older syntax. But, this will be useful when the db version is
> guaranteed to be recent. Thanks also for your reminder.

The one thing returning makes easy is getting the return id of multiple records.

insert into tablea values (val),(val),(val),(val) returning id;

will return a set of 4 different ids, not just one.  And if there's a
gap in the sequence you'll not get the wrong numbers.