Re: unlogged sequences - Mailing list pgsql-hackers

Patch rebased over some conflicts, and some tests simplified.

On 24.03.22 14:10, Peter Eisentraut wrote:
> Here is an updated patch that now also includes SET LOGGED/UNLOGGED 
> support.  So this version addresses all known issues and open problems.
> 
> 
> On 28.02.22 10:56, Peter Eisentraut wrote:
>> rebased patch, no functional changes
>>
>> On 11.02.22 10:12, Peter Eisentraut wrote:
>>> On 25.06.19 20:37, Andres Freund wrote:
>>>> I.e. I think it'd be better if we just added a fork argument to
>>>> fill_seq_with_data(), and then do something like
>>>>
>>>> smgrcreate(srel, INIT_FORKNUM, false);
>>>> log_smgrcreate(&rel->rd_node, INIT_FORKNUM);
>>>> fill_seq_with_data(rel, tuple, INIT_FORKNUM);
>>>>
>>>> and add a FlushBuffer() to the end of fill_seq_with_data() if writing
>>>> INIT_FORKNUM. The if (RelationNeedsWAL(rel)) would need an || 
>>>> forkNum ==
>>>> INIT_FORKNUM.
>>>
>>> Now that logical replication of sequences is nearing completion, I 
>>> figured it would be suitable to dust off this old discussion on 
>>> unlogged sequences, mainly so that sequences attached to unlogged 
>>> tables can be excluded from replication.
>>>
>>> Attached is a new patch that incorporates the above suggestions, with 
>>> some slight refactoring.  The only thing I didn't/couldn't do was to 
>>> call FlushBuffers(), since that is not an exported function.  So this 
>>> still calls FlushRelationBuffers(), which was previously not liked. 
>>> Ideas welcome.
>>>
>>> I have also re-tested the crash reported by Michael Paquier in the 
>>> old discussion and added test cases that catch them.
>>>
>>> The rest of the patch is just documentation, DDL support, client 
>>> support, etc.
>>>
>>> What is not done yet is support for ALTER SEQUENCE ... SET 
>>> LOGGED/UNLOGGED.  This is a bit of a problem because:
>>>
>>> 1. The new behavior is that a serial/identity sequence of a new 
>>> unlogged table is now also unlogged.
>>> 2. There is also a new restriction that changing a table to logged is 
>>> not allowed if it is linked to an unlogged sequence.  (This is IMO 
>>> similar to the existing restriction on linking mixed logged/unlogged 
>>> tables via foreign keys.)
>>> 3. Thus, currently, you can't create an unlogged table with a 
>>> serial/identity column and then change it to logged.  This is 
>>> reflected in some of the test changes I had to make in 
>>> alter_table.sql to work around this.  These should eventually go away.
>>>
>>> Interestingly, there is grammar support for ALTER SEQUENCE ... SET 
>>> LOGGED/UNLOGGED because there is this:
>>>
>>>          |   ALTER SEQUENCE qualified_name alter_table_cmds
>>>                  {
>>>                      AlterTableStmt *n = makeNode(AlterTableStmt);
>>>                      n->relation = $3;
>>>                      n->cmds = $4;
>>>                      n->objtype = OBJECT_SEQUENCE;
>>>                      n->missing_ok = false;
>>>                      $$ = (Node *)n;
>>>                  }
>>>
>>> But it is rejected later in tablecmds.c.  In fact, it appears that 
>>> this piece of grammar is currently useless because there are no 
>>> alter_table_cmds that actually work for sequences.  (This used to be 
>>> different because things like OWNER TO also went through here.)
>>>
>>> I tried to make tablecmds.c handle sequences as well, but that became 
>>> messy.  So I'm thinking about making ALTER SEQUENCE ... SET 
>>> LOGGED/UNLOGGED an entirely separate code path and rip out the above 
>>> grammar, but that needs some further pondering.
>>>
>>> But all that is a bit of a separate effort, so in the meantime some 
>>> review of the changes in and around fill_seq_with_data() would be 
>>> useful.