using the nextval('sequence_name') in sql, the result maybe is not right

On Wed, 26 Sep 2018 at 14:08, Wanglin <jluwln@163.com> wrote:

Hi, all:
PostgreSQL version : 10.3. I use "nextval" in the sql , but I think the result is not right, maybe it is a bug.
The test case as bellow:
create sequence seq1;
select nextval('seq1');
create table tx1(id1 int, id2 int);
insert into tx1 select generate_series(1,100), random()*102;
explain verbose select * from tx1 where id2 = nextval('seq1');;
select * from tx1 where id2 = nextval('seq1');
postgres=# explain verbose select * from tx1 where id2 = nextval('seq1');; QUERY PLAN ------------------------------------------------------------ Seq Scan on public.tx1 (cost=0.00..43.90 rows=11 width=8) Output: id1, id2 Filter: (tx1.id2 = nextval('seq1'::regclass)) (3 rows)

postgres=# select * from tx1 where id2 = nextval('seq1'); -- here, may be the result is not right id1 | id2 -----+----- 56 | 57 (1 row)

:: I think "nextval('seq1')" equal 2, so " select * from tx1 where id2 = nextval('seq1')" equals " select * from tx1 where id2 = 2", is it ?

No. nextval("seq1") increments the sequence and returns the new value.
It never[*] returns the same value in subsequent calls, that is the
purpose of the function (and sequences in general).

Normally, you would assign a sequence to a surrogate key field in your
table, so that you automatically get unique values in that field
(unless you mess around).
That's not how you're using it, so I wonder what your purpose is for
the sequence?

[*] Never is never without exceptions, just like always always has.
--
If you can't see the forest for the trees,
Cut the trees and you'll see there is no forest.

On 9/26/18 5:05 AM, Wanglin wrote:

Hi, all:
    PostgreSQL version : 10.3.  I use "nextval" in the sql , but I
think the result is not right, maybe it is a bug.
*The test case as bellow:*
create sequence seq1;
select nextval('seq1');
create table tx1(id1 int, id2 int);
insert into tx1 select generate_series(1,100), random()*102;
explain verbose select * from tx1 where id2 = nextval('seq1');;
select * from tx1 where id2 = nextval('seq1');
postgres=# explain verbose select * from tx1 where id2 =
nextval('seq1');; QUERY PLAN
------------------------------------------------------------ Seq Scan on
public.tx1 (cost=0.00..43.90 rows=11 width=8) Output: id1, id2 Filter:
(tx1.id2 = nextval('seq1'::regclass)) (3 rows)

postgres=# select * from tx1 where id2 = nextval('seq1'); *-- here,
**may be the result is not right* id1 | id2 -----+----- 56 | 57 (1 row)

:: I think "nextval('seq1')" equal 2, so "select * from tx1 where id2 =
nextval('seq1')" equals "select * from tx1 where id2 = 2", is it ?

As Alban pointed out calling nextval() increments the sequence. As your
EXPLAIN shows Postgres your SELECT is doing a sequence scan. Using your
test code here I get:

select * from tx1;

id1 | id2
-----+-----
1 | 27
2 | 42
3 | 93
4 | 2
5 | 85

So going in sequence Postgres is going to compare 27 to nextval()(which
is 2), not find it move to 42 = nextval()(=3) not find it and so on.

If I do:

select * from tx1 order by id2;

I get:

id1 | id2
-----+-----
20 | 0

<values removed for clarity>
2 | 42
17 | 43
63 | 45
88 | 45
27 | 46
52 | 47
47 | 47

alter sequence seq1 restart;

select * from tx1 where id2 = nextval('seq1') order by id2;
id1 | id2
-----+-----
47 | 47

The sequence catches up with the values because there are duplicate 47
values in id2.

Thanks,
    Wanglin

--
Adrian Klaver
adrian.klaver@aklaver.com