Re: [GENERAL] performance issue using DBI

If you need to get the actual "current value" without incrementing, try:

SELECT last_value FROM <sequence_name>;

This will return the last value returned from a nextval command directly

from the sequence properties in the system tables.

-----Original Message-----
From: Dan Langille [mailto:dan(at)langille(dot)org]
Sent: Thursday, June 06, 2002 12:34 PM
To: Nicolas Nolst
Cc: pgsql-admin(at)postgresql(dot)org; pgsql-general(at)postgresql(dot)org
Subject: Re: [ADMIN] [GENERAL] performance issue using DBI

On 6 Jun 2002 at 17:14, Oliver Elphick wrote:

> On Thu, 2002-06-06 at 14:55, Joshua b. Jore wrote:u
> > Don't use currval since some other process might alter the
> > sequence between the time you call nextval and currval.
>
> This is wrong. currval() will always return the last serial assigned
in
> *the*same*session*. It is not affected by other users at all.

Folks, here's a short test which might help. Note the BEGIN.

$ psql testing
testing=# create sequence test;
CREATE
testing=# select currval('test');
ERROR: test.currval is not yet defined in this session
testing=# select setval('test', 1);
setval
--------
1
(1 row)

testing=# select currval('test');
currval
---------
1
(1 row)

testing=# select currval('test');
currval
---------
1
(1 row)

testing=# select currval('test');
currval
---------
1
(1 row)

testing=# select currval('test');
currval
---------
1
(1 row)

testing=#

Then, in another window, I did this:

$ psql testing
# select nextval('test');
nextval
---------
2
(1 row)

testing=# select nextval('test');
nextval
---------
3
(1 row)

testing=# select nextval('test');
nextval
---------
4
(1 row)

testing=#

Then back to the other window:

testing=# select currval('test');
currval
---------
1
(1 row)

testing=# select nextval('test');
nextval
---------
5
(1 row)

testing=#

cheers

FWIW: I always use nextval when looking for a new ID.
--
Dan Langille

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