Re: unlogged sequences

On Thu, Mar 31, 2022 at 8:44 PM David G. Johnston
<david.g.johnston@gmail.com> wrote:

> The "give the user power" argument is also valid.  But since they already have power through unowned sequences, having the owned sequences more narrowly defined doesn't detract from usability, and in many ways enhances it by further reinforcing the fact that the sequence internally used when you say "GENERATED ALWAYS AS IDENTITY" is an implementation detail - one that has the same persistence as the table.

I think there's a question about what happens in the GENERATED ALWAYS
AS IDENTITY case. The DDL commands that create such sequences are of
the form ALTER TABLE something ALTER COLUMN somethingelse GENERATED
ALWAYS AS (sequence_parameters), and if we need to specify somewhere
in the whether the sequence should be logged or unlogged, how do we do
that?

Consider:

rhaas=# create unlogged table xyz (a int generated always as identity);
CREATE TABLE
rhaas=# \d+ xyz
                                                 Unlogged table "public.xyz"
 Column |  Type   | Collation | Nullable |           Default
 | Storage | Compression | Stats target | Description
--------+---------+-----------+----------+------------------------------+---------+-------------+--------------+-------------
 a      | integer |           | not null | generated always as
identity | plain   |             |              |
Access method: heap

rhaas=# \d+ xyz_a_seq
                     Sequence "public.xyz_a_seq"
  Type   | Start | Minimum |  Maximum   | Increment | Cycles? | Cache
---------+-------+---------+------------+-----------+---------+-------
 integer |     1 |       1 | 2147483647 |         1 | no      |     1
Sequence for identity column: public.xyz.a

In this new system, does the user still get a logged sequence?

If they
get an unlogged sequence, how does dump-and-restore work?

What if they
want to still have a logged sequence?