Re: DB migration: Loading a massive fixture dump

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On 2/09/2020 3:51 am, Fred Blaise wrote:
> Hi,
>
> I have to migrate a database from mysql to postgresql. Unfortunately,
> there is quite a bit of data in there.
>
> As a strategy, I thought of using Django's ORM feature, dump the data
> from my setup using mysql, and load the fixtures on the other setup
> using postgresql, such as:
>
> # on mysql
> $ docker exec -ti madchap_ddojo_uwsgi_1 bash -c 'python manage.py
> dumpdata --indent=4 -e sessions -e admin --natural-foreign
> --natural-primary -e contenttypes -e auth.permission -v2' >
> ~/Downloads/datadump_orm_dump_1.json
>
> This yields a 4GB json file.
>
> Reload on the other side, such as:
>
> # on postgresql
> $  docker exec -ti madchap_ddojo_uwsgi_1 bash -c 'python manage.py
> loaddata datadump_orm_dump_small.json'
>
> I let the loaddata run for 4hours, and it eventually returns with no
> error -- but no success either. I actually now think that the message
> indicating "Installed x object(s) from 1 fixture(s)" does not show, I
> will see again.
>
> Looking at the postgresql side, querying metatables such as:
>
> SELECT nspname || '.' || relname AS "relation",
>     pg_size_pretty(pg_total_relation_size(C.oid)) AS "total_size"
>   FROM pg_class C
>   LEFT JOIN pg_namespace N ON (N.oid = C.relnamespace)
>   WHERE nspname NOT IN ('pg_catalog', 'information_schema')
>     AND C.relkind <> 'i'
>     AND nspname !~ '^pg_toast'
>   ORDER BY pg_total_relation_size(C.oid) DESC
>   LIMIT 5;
>
> tells me loads of data gets in there. a "\d+" does not agree, however
> (weird, but I don't know enough to get the difference between the
> two). And yet, I cannot see anything through any SELECT statement
> (hinting to the transaction isolation).
>
> All things equal, this strategy works fine when working with small
> json files. I just successfully did it with a file less than 1MB.
>
> I am guessing I have a few questions, I am stuck here.
>
> 1. Are there any hidden args/switches to manage.py loaddata or else
> that I could look at?

loaddata is intended for small imports such as skeleton data for unit
testing


> 2. Are there specific django configuration that could help me load the
> data?
> 3. Does a loaddata actually result in a massive single transaction?
> (looks like so, sounds crazy to me :))

Probably does because that would be what you most likely want. But as
you have discovered it probably runs out of resources if the load is too
big.

> 4. Is there a better way to migrate my data?

Yes.

https://github.com/lanyrd/mysql-postgresql-converter/blob/master/db_converter.py

I used this successfully a few months ago. I had to convert it from
py2.7 to 3.x.

Briefly, you will have to dump MySQL again using a similar incantation
to this  ...

mysqldump --compatible=postgresql --default-character-set=utf8 -r
databasename.mysql -u root databasename

If you (or anyone) would like a copy of my adjusted db_converter.py,
drop me a line off-list.

Cheers

Mike

> Thanks a lot for any pointers!
> Cheers.
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