Re: documentation clarifications for "alter sequence" ? - Mailing list pgsql-docs

I looked into this report.  When we do a CREATE or ALTER sequence, we
check that the new sequence row is valid.  We don't track which values
were changed by ALTER, which is why you are seeing this kind of error
message wording.  I don't think it is worth tracking this to generate
clearer error messages since this is the first complaint I have seen
about this.

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On Thu, Nov 21, 2019 at 08:06:28PM +0000, PG Doc comments form wrote:
> The following documentation comment has been logged on the website:
> 
> Page: https://www.postgresql.org/docs/12/sql-altersequence.html
> Description:
> 
> postgresql 11.5
> 
> I execute this:
> CREATE TABLE v5processing.just_testing(
>     id SERIAL,
>     some_test text
> );
> 
> -- sequence last value is now 0, min value is 1
> 
> then I execute this:
> 
> insert into v5processing.just_testing(some_test) values ('data');
> insert into v5processing.just_testing(some_test) values ('more data');
> insert into v5processing.just_testing(some_test) values ('whatever');
> 
> -- last value is now 3, min value is 1
> 
> But alter sequence behaves thusly:
> 
> alter sequence just_testing_id_seq minvalue 4 -- ERROR: START value (1)
> cannot be less than MINVALUE (4)
> 
> -- it would be helpful if this interaction/ relation were explicitly pointed
> out, and why it behaves this way. ("MINVALUE cannot be greater than START
> value" makes more sense to me, since minvalue is the thing I'm attempting to
> alter. Or even "START value must be at least MINVALUE", if in fact that's
> the limitation.)
> 
> alter sequence just_testing_id_seq restart with 4 -- no error, but still has
> min value 1
> 
> --  "Any parameters not specifically set in the ALTER SEQUENCE command
> retain their prior settings". OK, but this does not seem intuitive, at least
> to me. What meaning or use does a min value have after a restart?
> 
> alter sequence just_testing_id_seq minvalue 4 -- ERROR: START value (1)
> cannot be less than MINVALUE (4)
> 
> -- clearly my restart had no visible effect
> 
> But:
> 
> alter sequence just_testing_id_seq restart with 2 -- no error , but still
> has min value 1 and last value 3
> alter sequence just_testing_id_seq minvalue 2; -- ERROR: START value (1)
> cannot be less than MINVALUE (2)
> 
> On the other hand :
> alter sequence just_testing_id_seq  start 2;
> 
> followed by
> alter sequence just_testing_id_seq minvalue 2;
> 
> throws no error, which leads me to believe there is a subtle difference
> between restart and start. But lastvalue is unchanged, and inserting a new
> record did not apparently start or restart at 2 since that should have
> triggered an error(?). Instead the insert succeeded with id = 4.
> 
> I stumbled on this when trying to reset a test DB to all sequences starting
> at 1, and finding that there was one the had somehow gotten a start value of
> 6. I would have expected that 'restart 1' did just that, with no
> complications. Maybe I'm the only one who's confused.
> 
> Also, "setval function" is mentioned. A link to that would be helpful.
> Thanks.


-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

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