Re: ALTER tbl rewrite loses CLUSTER ON index

On Thu, Apr 02, 2020 at 03:14:21PM +0900, Michael Paquier wrote:
> Now, regarding patch 0002, note that you have a problem for this part:
> -            tuple = SearchSysCache1(INDEXRELID, ObjectIdGetDatum(indexOid));
> -            if (!HeapTupleIsValid(tuple))    /* probably can't happen */
> -            {
> -                relation_close(OldHeap, AccessExclusiveLock);
> -                pgstat_progress_end_command();
> -                return;
> -            }
> -            indexForm = (Form_pg_index) GETSTRUCT(tuple);
> -            if (!indexForm->indisclustered)
> +            if (!get_index_isclustered(indexOid))
>              {
> -                ReleaseSysCache(tuple);
>                  relation_close(OldHeap, AccessExclusiveLock);
>                  pgstat_progress_end_command();
>                  return;
>              }
> -            ReleaseSysCache(tuple);
> 
> On an invalid tuple for pg_index, the new code would issue an error,
> while the old code would just return.  And it seems to me that this
> can lead to problems because the parent relation is processed and
> locked at the beginning of cluster_rel(), *after* we know the index
> OID to work on.

> The refactoring is fine for the other two areas
> though, so I think that there is still value to apply
> get_index_isclustered() within mark_index_clustered() and cluster(),
> and I would suggest to keep 0002 to that.
> 
> Justin, what do you think?

Sounds right.  Or else get_index_isclustered() could be redefined to take a
boolean "do_elog" flag, and if syscache fails and that's false, then return
false instead of ERROR.

-- 
Justin