Re: nextval() on serial using old, existing value on insert?

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От Keith
Тема Re: nextval() on serial using old, existing value on insert?
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Msg-id CAHw75vtvE1o1YqA65hYUy9y4dV69VAXwMrEORhcK7SLbmE6CZA@mail.gmail.com
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Ответ на nextval() on serial using old, existing value on insert?  (Wells Oliver <wells.oliver@gmail.com>)
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On Thu, May 11, 2023 at 12:19 AM Wells Oliver <wells.oliver@gmail.com> wrote:
I have a simple table with a given column defined like so:

 common_key    | integer |           | not null | nextval('alias.identity_common_key_seq'::regclass)  | plain


Very very very infrequently, on an INSERT where this column is not specified, this column will be assigned a value that already exists in the table, versus the next presumably unused value in the sequence. I cannot figure this out. Is there any reason why this might be the case?

--

Seeing the complete schema of your table would help with clarity. Can you share the full \d+ output of it if the below doesn't answer your question?

When using a standard sequence as a default value, unless you have some other restrictions on the table, someone could still manually insert a value into the table that is ahead of the current sequence value. Then when someone tries to do an insert without specifying the column, it tries to use the next available value which eventually hits the value someone else inserted. Also assuming you have some sort of unique index on this column, that would then cause a constraint violation.

If you need to enforce that the sequence values are the only ones allowed for this column, I would recommend looking into the IDENTITY property for table columns. This allows you to enforce that the column's value can only be obtained from a sequence.

Adjusted example from the documentation (https://www.postgresql.org/docs/15/sql-createtable.html) to enforce sequence only values (changed GENERATED BY DEFAULT to GENERATED ALWAYS)
CREATE TABLE distributors (     did    integer PRIMARY KEY GENERATED ALWAYS AS IDENTITY,     name   varchar(40) NOT NULL CHECK (name <> '')
);

 \d+ distributors
                                                        Table "public.distributors"
 Column |         Type          | Collation | Nullable |           Default            | Storage  | Compression | Stats target | Description
--------+-----------------------+-----------+----------+------------------------------+----------+-------------+--------------+-------------
 did    | integer               |           | not null | generated always as identity | plain    |             |              |
 name   | character varying(40) |           | not null |                              | extended |             |              |
Indexes:
    "distributors_pkey" PRIMARY KEY, btree (did)
Check constraints:
    "distributors_name_check" CHECK (name::text <> ''::text)
Access method: heap

Hope that helps!

Keith

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