Re: Bug in pg_dump

On 1/15/15 5:26 AM, Gilles Darold wrote:
> Hello,
>
> There's a long pending issue with pg_dump and extensions that have table members with foreign keys. This was
previouslyreported in this thread
http://www.postgresql.org/message-id/CA+TgmoYVZkAdMGh_8EL7UVM472GerU0b4pnNFjQYe6ss1K9wDQ@mail.gmail.comand discuss by
Robert.All PostgreSQL users that use the PostGis extension postgis_topology are facing the issue because the two
memberstables (topology and layer) are linked by foreign keys.
 
>
> If you dump a database with this extension and try to import it you will experience this error:
>
>     pg_restore: [archiver (db)] Error while PROCESSING TOC:
>     pg_restore: [archiver (db)] Error from TOC entry 3345; 0 157059176 TABLE DATA layer gilles
>     pg_restore: [archiver (db)] COPY failed for table "layer": ERROR: insert or update on table "layer" violates
foreignkey constraint "layer_topology_id_fkey"
 
>     DETAIL:  Key (topology_id)=(1) is not present in table "topology".
>     WARNING: errors ignored on restore: 1
>
>
> The problem is that, whatever export type you choose (plain/custom and full-export/data-only) the data of tables
"topology"and "layer" are always exported in alphabetic order. I think this is a bug because outside extension, in
data-onlyexport, pg_dump is able to find foreign keys dependency and dump table's data in the right order but not with
extension'smembers. Default is alphabetic order but that should not be the case with extension's members because
constraintsare recreated during the CREATE EXTENSION order. I hope I am clear enough.
 
>
> Here we have three solutions:
>
>      1/ Inform developers of extensions to take care to alphabetical order when they have member tables using foreign
keys.
>      2/ Inform DBAs that they have to restore the failing table independently. The use case above can be resumed
usingthe following command:
 
>
>           pg_restore -h localhost -n topology -t layer -Fc -d testdb_empty testdump.dump
>
>      3/ Inform DBAs that they have to restore the schema first then the data only using --disable-triggers

I don't like 1-3, and I doubt anyone else does...

>      4/ Patch pg_dump to solve this issue.

5. Disable FK's during load.
This is really a bigger item than just extensions. It would have the nice benefit of doing a wholesale FK validation
insteadof firing per-row triggers, but it would leave the database in a weird state if a restore failed...
 

> I attach a patch that solves the issue in pg_dump, let me know if it might be included in Commit Fest or if the three
othersolutions are a better choice. I also join a sample extension (test_fk_in_ext) to be able to reproduce the issue
andtest the patch. Note that it might exists a simpler solution than the one I used in this patch, if this is the case
pleasepoint me on the right way, I will be pleased to rewrite and send an other patch.
 

The only problem I see with this approach is circular FK's:

decibel@decina.local=# create table a(a_id serial primary key, b_id int);
CREATE TABLE
decibel@decina.local=# create table b(b_id serial primary key, a_id int references a);
CREATE TABLE
decibel@decina.local=# alter table a add foreign key(b_id) references b;
ALTER TABLE
decibel@decina.local=#

That's esoteric enough that I think it's OK not to directly support them, but pg_dump shouldn't puke on them (and
reallyshould throw a warning). Though it looks like it doesn't handle that in the data-only case anyway...
 
-- 
Jim Nasby, Data Architect, Blue Treble Consulting
Data in Trouble? Get it in Treble! http://BlueTreble.com