duplicate key violates unique constraint

List,

We're using 8.2.1 in RedHat ES 4 here and I have a simple two column
table: data_keys1.

Table "public.data_keys1"
Column | Type | Modifiers
--------+--------+------------------------------------------------------
---
id | bigint | not null default
nextval('data_keys1_id_seq'::regclass)
key1 | text |
Indexes:
"data_keys1_pkey" PRIMARY KEY, btree (id)

I also have a C program using libpq that populates this table via a
PREPARE'd statement within a transaction. Recently, while this table
was being loaded, it encountered an error I hadn't seen before and is a
bit confusing.

2007-05-09 09:27:07 EDT [22853] : LOG: statement: EXECUTE
insertKey1('vgdisplay');
2007-05-09 09:27:07 EDT [22853] : DETAIL: prepare: PREPARE
insertKey1(text) AS INSERT INTO public.data_keys1 (key1) values ($1);
2007-05-09 09:27:07 EDT [22853] : ERROR: duplicate key violates unique
constraint "data_keys1_pkey"
2007-05-09 09:27:07 EDT [22853] : STATEMENT: EXECUTE
insertKey1('vgdisplay');

As you can see, it's only providing the key1 column and the sequence is
providing the value for the column with the constraint. How can this be
happening? There were no other transactions, commits, individual
inserts happenning at the same time or within several seconds of this
one.

I'd like to explore sequences a bit more b/c as they are implemented in
PostgreSQL is a little confusing. Normally, I'd start another thread
but it may have some bearing here.


Sequences...

Now, I don't know if this just hasn't been tested or is a documented
feature (a.k.a. bug) but something does not seem right here. In this
test case I'm able to get the same sequence ID's via two psql
connections to the same database on the same sequence.

Connect to "db" in two different psql sessions (I'll prefix them below
with 1: and 2:) and in one create the table

1: db=# CREATE TABLE t1 ( s serial, i int);
1: db=# \d t1
Table "public.t1"
Column | Type | Modifiers
--------+---------+------------------------------------------------
s | integer | not null default nextval('t1_s_seq'::regclass)
i | integer |

1: db=# SELECT * FROM t1_s_seq;
sequence_name | last_value | increment_by | max_value |
min_value | cache_value | log_cnt | is_cycled | is_called
---------------+------------+--------------+---------------------+------
-----+-------------+---------+-----------+-----------
t1_s_seq | 12 | 1 | 9223372036854775807 |
1 | 1 | 32 | f | t
(1 row)

So, cache on t1_s_seq is set to 1. Not sure why the max_value is so
high when the column was specified as a 'serial' not a 'serial8' but
perhaps I should check the code out and submit a patch for that. I
digress.

Check out the value for t1_s_seq on connection #1.

1: db=# select nextval('t1_s_seq');
nextval
---------
1
(1 row)

And check the value for t1_s_seq on connection #2.

2: db=# select nextval('t1_s_seq');
nextval
---------
2
(1 row)

So far, so good. Now start a transaction on connection #1, advance
t1_s_seq by 10 but don't commit;

1: db=# begin;
BEGIN
1: db=# select setval('t1_s_seq', currval('t1_s_seq')+10);
setval
--------
11
(1 row)

And check the current value for t1_s_seq on connection #2.

2: db=# select currval('t1_s_seq');
nextval
---------
2
(1 row)


That's expected since the transaction on connection #1 hasn't been
commited. Now commit the transaction on connection #1 and check it's
current value.

1: db=# commit;
COMMIT
1: db=# select currval('t1_s_seq');
currval
---------
11
(1 row)

Again, expected. Now let's check the current value on connection #2
again.

2: db=# select currval('t1_s_seq');
currval
---------
2
(1 row)

This is where I take issue with the output. I'm not sure what benefit
cache value has as the client should, in this case, consulted with the
backend as too the value of the sequence.

Sequences are suppose to be unique but in this case it seems that may
not always be the case. Sequences have some kind of odd relationship
(no pun intended) with transactions in that they are in some cases in
sync regardless of the connection or query and in other situations, such
as above, are out of sync.

Just for fun, select on connection #2 the nextval of the sequence.

2: db=# select nextval('t1_s_seq');
nextval
---------
12
(1 row)

*boogle*

What have I done wrong here? Does it have any bearing on my unique
constraint error? I wouldn't think so but I haven't found any other
possible explanation.

TIA,
Greg

--
Greg Spiegelberg
Manager, Product Development
ISOdx Solutions, a division of Cranel, Inc.
gspiegelberg(at)isodxsolutions(dot)com
<mailto:gspiegelberg(at)isodxsolutions(dot)com>
614.318.4314, office
614.431.8388, fax