Re: Disabling Heap-Only Tuples

On Thu, 2023-07-06 at 22:18 +0200, Matthias van de Meent wrote:
> On Wed, 5 Jul 2023 at 19:55, Thom Brown <thom(at)linux(dot)com> wrote:
> >
> > On Wed, 5 Jul 2023 at 18:05, Matthias van de Meent
> > <boekewurm+postgres(at)gmail(dot)com> wrote:
> > > So what were you thinking of? A session GUC? A table option?
> >
> > Both.
>
> Here's a small patch implementing a new table option max_local_update
> (name very much bikesheddable). Value is -1 (default, disabled) or the
> size of the table in MiB that you still want to allow to update on the
> same page. I didn't yet go for a GUC as I think that has too little
> control on the impact on the system.
>
> I decided that max_local_update would be in MB because there is no
> reloption value that can contain MaxBlockNumber and -1/disabled; and 1
> MiB seems like enough granularity for essentially all use cases.
>
> The added regression tests show how this feature works, that the new
> feature works, and validate that lock levels are acceptable
> (ShareUpdateExclusiveLock, same as for updating fillfactor).

I have looked at your patch, and I must say that I like it. Having
a size limit is better than my original idea of just "on" or "off".
Essentially, it is "try to shrink the table if it grows above a limit".

The patch builds fine and passes all regression tests.

Documentation is missing.

I agree that the name "max_local_update" could be improved.
Perhaps "avoid_hot_above_size_mb".

--- a/src/include/utils/rel.h
+++ b/src/include/utils/rel.h
@@ -342,6 +342,7 @@ typedef struct StdRdOptions
int parallel_workers; /* max number of parallel workers */
StdRdOptIndexCleanup vacuum_index_cleanup; /* controls index vacuuming */
bool vacuum_truncate; /* enables vacuum to truncate a relation */
+ int max_local_update; /* Updates to pages after this block must go through the VM */
} StdRdOptions;

#define HEAP_MIN_FILLFACTOR 10

In the comment, it should be FSM, not VM, right?

Other than that, I see nothing wrong.

Yours,
Laurenz Albe