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

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(dot)klaver(at)aklaver(dot)com