Re: row filtering for logical replication

On Thu, Jan 20, 2022 at 4:54 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:

> + /*
> + * Unchanged toasted replica identity columns are only detoasted in the
> + * old tuple, copy this over to the new tuple.
> + */
> + if (att->attlen == -1 &&
> + VARATT_IS_EXTERNAL_ONDISK(tmp_new_slot->tts_values[i]) &&
> + !VARATT_IS_EXTERNAL_ONDISK(old_slot->tts_values[i]))
> + {
> + if (tmp_new_slot == new_slot)
> + {
> + tmp_new_slot = MakeSingleTupleTableSlot(desc, &TTSOpsVirtual);
> + ExecClearTuple(tmp_new_slot);
> + ExecCopySlot(tmp_new_slot, new_slot);
> + }
> +
> + tmp_new_slot->tts_values[i] = old_slot->tts_values[i];
> + tmp_new_slot->tts_isnull[i] = old_slot->tts_isnull[i];
> + }
> + }
>
>
> What is the need to assign new_slot to tmp_new_slot at the beginning
> of this part of the code? Can't we do this when we found some
> attribute that needs to be copied from the old tuple?
>
> The other part which is not clear to me by looking at this code and
> comments is how do we ensure that we cover all cases where the new
> tuple doesn't have values?
>

IMHO, the only part we are trying to handle is when the toasted attribute
is not modified in the new tuple. And if we notice the update WAL the new
tuple is written as it is in the WAL which is getting inserted into the
heap page. That means if it is external it can only be in
VARATT_IS_EXTERNAL_ONDISK format. So I don't think we need to worry about
any intermediate format which we use for the in-memory tuples. Sometimes
in reorder buffer we do use the INDIRECT format as well which internally
can store ON DISK format but we don't need to worry about that case either
because that is only true when we have the complete toast tuple as part of
the WAL and we recreate the tuple in memory in reorder buffer, so even if
it can by ON DISK format inside INDIRECT format but we have complete tuple.

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com