Re: Sequence Access Methods, round two

On Tue, Feb 27, 2024 at 10:27:13AM +0900, Michael Paquier wrote:
> On Thu, Feb 22, 2024 at 05:36:00PM +0100, Tomas Vondra wrote:
>> 0001
>> ------
>>
>> I think this bit in pg_proc.dat is not quite right:
>>
>>   proallargtypes => '{regclass,bool,int8}', proargmodes => '{i,o,o}',
>>   proargnames => '{seqname,is_called,last_value}',
>>
>> the first argument should not be "seqname" but rather "seqid".
>
> Ah, right.  There are not many system functions that use regclass as
> arguments, but the existing ones refer more to IDs, not names.

This patch set is not going to be merged for this release, so I am
going to move it to the next commit fest to continue the discussion in
v18~.

Anyway, there is one piece of this patch set that I think has a lot of
value outside of the discussion with access methods, which is to
redesign pg_sequence_last_value so as it returns a (last_value,
is_called) tuple rather than a (last_value).  This has the benefit of
switching pg_dump to use this function rather than relying on a scan
of the heap table used by a sequence to retrieve the state of a
sequence dumped.  This is the main diff:
-    appendPQExpBuffer(query,
-                      "SELECT last_value, is_called FROM %s",
-                      fmtQualifiedDumpable(tbinfo));
+    /*
+     * In versions 17 and up, pg_sequence_last_value() has been switched to
+     * return a tuple with last_value and is_called.
+     */
+    if (fout->remoteVersion >= 170000)
+        appendPQExpBuffer(query,
+                          "SELECT last_value, is_called "
+                          "FROM pg_sequence_last_value('%s')",
+                          fmtQualifiedDumpable(tbinfo));
+    else
+        appendPQExpBuffer(query,
+                          "SELECT last_value, is_called FROM %s",
+                          fmtQualifiedDumpable(tbinfo));

Are there any objections to that?  pg_sequence_last_value() is
something that we've only been relying on internally for the catalog
pg_sequences.
--
Michael