Re: pglogical vs. built-in logical replication in pg-10

On 22/06/2017 17:46, Andreas Joseph Krogh wrote:
> På torsdag 22. juni 2017 kl. 15:25:20, skrev Achilleas Mantzios <achill(at)matrix(dot)gatewaynet(dot)com <mailto:achill(at)matrix(dot)gatewaynet(dot)com>>:
>
> On 22/06/2017 13:38, Andreas Joseph Krogh wrote:
>> På torsdag 22. juni 2017 kl. 11:43:02, skrev Achilleas Mantzios <achill(at)matrix(dot)gatewaynet(dot)com <mailto:achill(at)matrix(dot)gatewaynet(dot)com>>:
>>
>> On 22/06/2017 11:21, Andreas Joseph Krogh wrote:
>>> Hi.
>>> 1. Why should one prefer built-in logical replication in pg-10 to pglogical, does it do anything pglogical doesn't?
>>> It seems pglogical is more feature-rich...
>>> 2. As I understand built-in logical replication in pg-10 doesn't support large-objects, which we use a lot. Does pglogical replicate large objects? I cannot find any notes about
>>> large-objects under "Limitations and Restrictions": https://www.2ndquadrant.com/en/resources/pglogical/pglogical-docs/
>> You may do a simple test, create a table with a largeobject and try to read the logical stream, if it cannot represent the lo_import, lo_open, lowrite, lo_close (and I 'd bet they can't be
>> encoded) then neither pglogical (being based on the same logical decoding technology) will support them
>>
>> The point of email-lists like this is that one may share knowledge so one doesn't have to test everything one self, and can build on knowledge from others. I'm looking for an answer from
>> someone who's not betting, but knows.
> I gave you enough knowledge already. Here's some more :
> - go and install 10
> - create a table containing one col with type oid (large object) and one bytea
> - follow the simple setup here : https://www.postgresql.org/docs/10/static/logicaldecoding-example.html
> - insert a row
> - Do again : SELECT * FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL);
>
> Do you see any of your oid image data in the output? Do you see any of the bytea ? (the answer here in 9.5 is "no"/"yes").
> If in 10.0 is still the case, then you should think about moving to bytea.
>
> Hm, it turns out it's not quite that simple...
> Test-case:
> create table drus(id bigint primary key, lo oid, data bytea);
> SELECT * FROM pg_create_logical_replication_slot('my_slot', 'test_decoding');
> INSERT INTO drus (id, lo, data) values(1, lo_import('/tmp/faktura_200007.pdf'), decode('AAAEEE', 'hex'));
>
> select * from drus;
> ┌────┬─────────┬──────────┐
> │ id │ lo │ data │
> ├────┼─────────┼──────────┤
> │ 1 │ 2873269 │ \xaaaeee │
> └────┴─────────┴──────────┘
>
>
> SELECT * FROM pg_logical_slot_get_changes('my_slot', NULL, NULL);
> ┌────────────┬──────┬────────────────────────────────────────────────────────────────────────────────┐
> │ lsn │ xid │ data │
> ├────────────┼──────┼────────────────────────────────────────────────────────────────────────────────┤
> │ B/E585B858 │ 9391 │ BEGIN 9391 │
> │ B/E586BE78 │ 9391 │ table public.drus: INSERT: id[bigint]:1 lo[oid]:2873269 data[bytea]:'\xaaaeee' │
> │ B/E586BF80 │ 9391 │ COMMIT 9391 │
> └────────────┴──────┴────────────────────────────────────────────────────────────────────────────────┘
> (3 rows)
>
> So far so good, the oid-value (2873269) is apparently in the change-set, but...
If the data itself of the LO are not there then this is not so good.
> Set up publication:
> CREATE PUBLICATION bolle FOR ALL TABLES;
> CREATE PUBLICATION
>
> === ON REPLICA ===
> # create table on replica:
> create table drus(id bigint primary key, lo oid, data bytea);
> # create subscription:
> CREATE SUBSCRIPTION mysub CONNECTION 'host=localhost port=5433 user=andreak dbname=fisk' PUBLICATION bolle;
> NOTICE: created replication slot "mysub" on publisher
> CREATE SUBSCRIPTION
> 2017-06-22 16:38:34.740 CEST [18718] LOG: logical replication apply worker for subscription "mysub" has started
> 2017-06-22 16:38:34.747 CEST [18720] LOG: logical replication table synchronization worker for subscription "mysub", table "drus" has started
> 2017-06-22 16:38:35.746 CEST [18720] LOG: logical replication table synchronization worker for subscription "mysub", table "drus" has finished
>
> Looks good:
> select * from drus;
> ┌────┬─────────┬──────────┐
> │ id │ lo │ data │
> ├────┼─────────┼──────────┤
> │ 1 │ 2873269 │ \xaaaeee │
> └────┴─────────┴──────────┘
> (1 row)
>
> ...until :
> SELECT lo_export(drus.lo, '/tmp/faktura.pdf') from drus where id = 1;
> 2017-06-22 16:40:04.967 CEST [18657] ERROR: large object 2873269 does not exist
> 2017-06-22 16:40:04.967 CEST [18657] STATEMENT: SELECT lo_export(drus.lo, '/tmp/faktura.pdf') from drus where id = 1;
> ERROR: large object 2873269 does not exist
>
> So, the OID-value is replicated but pg_largeobject is empty:
> select * from pg_largeobject;
> ┌──────┬────────┬──────┐
> │ loid │ pageno │ data │
> ├──────┼────────┼──────┤
> └──────┴────────┴──────┘
> (0 rows)
>
> Once again having pg_largeobject as a system-catalog prevents LOs from working smoothly. Neither replication nor having LOs on a different tablespace (by moving pg_largeobject) works.
I think logical decoding was designed for supporting DML SQL commands (i.e. a finite set of commands) and not specific functions (lo_*) which by nature can be arbitrary, infinite and version specific.
> I wish PG in some future version will address these quirks so one can operate on LOs more smoothly.
What's so better in LO's VS bytea? You do a lot updates on the binary data, changing only parts of it?
> --
> *Andreas Joseph Krogh*
> CTO / Partner - Visena AS
> Mobile: +47 909 56 963
> andreas(at)visena(dot)com <mailto:andreas(at)visena(dot)com>
> www.visena.com <https://www.visena.com>
> <https://www.visena.com>

--
Achilleas Mantzios
IT DEV Lead
IT DEPT
Dynacom Tankers Mgmt