Re: Triggers on foreign tables

2013/9/10 Ronan Dunklau <rdunklau(at)gmail(dot)com>:
> For row-level triggers, it seems more complicated. From what I understand,
> OLD/NEW tuples are fetched from the heap using their ctid (except for BEFORE
> INSERT triggers). How could this be adapted for foreign tables ?
>
It seems to me the origin of difficulty to support row-level trigger
is that foreign table
does not have a back-door to see the older tuple to be updated, unlike
regular tables.
The core-PostgreSQL knows on-disk format to store tuples within
regular tables, thus,
GetTupleForTrigger() can fetch a tuple being identified by tupleid.
On the other hand, writable foreign table is also designed to identify
a remote tuple
with tupleid, as long as FDW module is responsible.
So, a straightforward idea is adding a new FDW interface to get an
older image of
the tuple to be updated. It makes possible to implement something similar to
GetTupleForTrigger() on foreign tables, even though it takes a
secondary query to
fetch a record from the remote server. Probably, it is an headache for us.

One thing we pay attention is, the tuple to be updated is already
fetched from the
remote server and the tuple being fetched latest is (always?) the
target of update
or delete. It is not a heavy job for ForeignScanState node to hold a
copy of the latest
tuple. Isn't it an available way to reference relevant
ForeignScanState to get the older
image?
If my assumption is right, all we need to enhance are (1) add a store on
ForeignScanState to hold the last tuple on the scan stage, (2) add
GetForeignTupleForTrigger() to reference the store above on the relevant
ForeignScanState.

Any comment please.
--
KaiGai Kohei <kaigai(at)kaigai(dot)gr(dot)jp>