Re: [Postgres-xc-developers] Replicated update and delete

[Amit K. <ami...@en...>]

On 6 November 2013 18:31, Michael Paquier <mic...@gm...> wrote:

> On Wed, Nov 6, 2013 at 3:28 PM, Amit Khandekar
> <ami...@en...> wrote:
> > What exactly does the PostgreSQL FDW doc say about updates and primary
> key ?
> By having a look here:
>
> http://www.postgresql.org/docs/9.3/static/fdw-callbacks.html#FDW-CALLBACKS-UPDATE
> It is recommended to use a kind of row ID or the primary key columns.
> In the case of XC row ID = CTID, and its uniqueness is not guaranteed
> except if coupled with a node ID, which I think it has... Using a CTID
> + node ID combination makes the analysis of tuple uniqueness
> impossible for replicated tables either way, so a primary key would be
> better IMO.
>
> > How does the postgres_fdw update a table that has no primary or unique
> key ?
> It uses the CTID when scanning remote tuples for UPDATE/DELETE, thing
> guarantying that tuples are unique in this case as the FDW deals with
> a single server, here is for example the case of 2 nodes listening
> ports 5432 and 5433.
> $ psql -p 5433 -c "CREATE TABLE aa (a int, b int);"
> CREATE TABLE
>
> On server with port 5432:
> =# CREATE EXTENSION postgres_fdw;
> CREATE EXTENSION
> =# CREATE SERVER postgres_server FOREIGN DATA WRAPPER postgres_fdw
> OPTIONS (host 'localhost', port '5432', dbname 'ioltas');
> CREATE SERVER
> =# CREATE USER MAPPING FOR PUBLIC SERVER postgres_server OPTIONS (password
> '');
> CREATE USER MAPPING
> =# CREATE FOREIGN TABLE aa_foreign (a int, b int) SERVER
> postgres_server OPTIONS (table_name 'aa');
> CREATE FOREIGN TABLE
> =# explain verbose update  aa_foreign set a = 1, b=2 where a = 1;
>                                    QUERY PLAN
>
> --------------------------------------------------------------------------------
>  Update on public.aa_foreign  (cost=100.00..144.40 rows=14 width=6)
>    Remote SQL: UPDATE public.aa SET a = $2, b = $3 WHERE ctid = $1
>    ->  Foreign Scan on public.aa_foreign  (cost=100.00..144.40 rows=14
> width=6)
>          Output: 1, 2, ctid
>          Remote SQL: SELECT ctid FROM public.aa WHERE ((a = 1)) FOR UPDATE
> (5 rows)
> And ctid is used for scanning...
>
> > In the patch, what do we do when the replicated table has no
> unique/primary
> > key ?
> I didn't look at the patch, but I think that replicated tables should
> also need a primary key. Let's imagine something like that with
> sessions S1 and S2 for a replication table, and 2 datanodes (1 session
> runs in common on 1 Coordinator and each Datanode):
> S1: INSERT VALUES foo in Dn1
> S2: INSERT VALUES foo2 in Dn1
> S2: INSERT VALUES foo2 in Dn2
> S1: INSERT VALUES foo in Dn2
> This will imply that those tuples have a different CTID, so a primary
> key would be necessary as I think that this is possible.
>

If the patch does not handle the case of replicated table without unique
key, I think we should have a common solution which takes care of this case
also. Or else, if this solution can be extended to handle no-unique-key
case, then that would be good. But I think we would end up in having two
different implementations, one for unique-key method, and another for the
other method, which does not seem good.

The method I had in mind was :
In the scan plan, fetch ctid, node_id from all the datanodes.  Use UPDATE
where ctd = ? , but use nodeid-based method to  generate the ExecNodes at
execute-time  (enhance ExecNodes->en_expr evaluation so as to use the
nodeid from source plan, as against the distribution column that it
currently uses for distributed tables) .
But this method will not work as-is in case of non-shippable row triggers.
Because trigger needs to be fired only once per row, and we are going to
execute UPDATE for all of the ctids of a given row corresponding to all of
the datanodes. So somehow we should fire triggers only once. This method
will also hit performance, because currently we fetch *all* columns and not
just ctid, so it's better to first do that optimization of fetching only
reqd columns (there's one pending patch submitted in the mailing list,
which fixes this).

This is just one approach, there might be better approaches.

Overall, I think if we decide to get this issue solved (and I think we
should really, this is a serious issue), sufficient resource time needs to
be given to think over and have discussions before we finalize the approach.


--
> Michael
>