Re: Creating complex track changes database - challenge!

Hi Lukasz

I am working on a generic (reading the information schema and other database metadata), trigger based solution for SCD tables, i. e. tables that keep (or not according to SCD type) history of the data. However, it is not far grown and I am not having much time to advance it so it evolves very slowly. If you are interested, I would open a sourceforge project or the like and we can work on it together.

I am very much surprised that no database I know of supports SCD/historising tables out of the box. In 16 years as ETL pro I have seen reinvented the wheel all the time... maybe PostgreSQL wants to get a head start on this.

Kind regards

Thiemo



Zitat von ?ukasz Jarych <jaryszek@gmail.com>:

Hi Guys,

I have idea already for creating this complex solution.

Please give your notes and tips if you have.

1. Keep all changes within table including:
-adding rows
-deleting
-editing

This can be managed by adding triggers and one additional table where you
can have sum up what was changed.

2. Changing DDL of tables:

I think that creating trigger for metadata should solve the problem. How
can i do it? I do not know already ...:)

3. Changing tables versioning.

It it is possible to save table (back up or something) to disc - i can
check the latest date of change and save table with this date and name.
And create table with all tables changes and version.
What do you think ?

4. Still problem with creating whole database versioning.
I found very interesting link but i not understand how it is works:

https://odetocode.com/blogs/scott/archive/2008/02/02/versioning-databases-change-scripts.aspx

Best,
Jacek

2018-02-26 12:16 GMT+01:00 ?ukasz Jarych <jaryszek@gmail.com>:

Hi Manual,
thank you very much!

Regarding your  tool - if it is not supported and it is for specific case
- i will not use it but figure out something new. I do not even how to
install this .hs files...

I thought about creating triggers to have all changes to specific tables.
And for each table name (or number) keep changes in one separate table.
What do you think about it?

If all you need is saving and restoring specific table states, logical

dumps with pg_dump should probably be enough for your needs.

Can you explain in details how can i use it?
What if user add new column? I can save ma table for example as version 3
and come back to version 1 in the future? (without this new column?)

Best,
Jacek



2018-02-26 12:04 GMT+01:00 Manuel Gómez <targen@gmail.com>:

On Mon, Feb 26, 2018 at 11:44 AM ?ukasz Jarych <jaryszek@gmail.com>

wrote:

I have to:

1. Keep all changes within table including:
-adding rows
-deleting
-editing

2. Save table with specific state and recover specific state (so go back
to previous table versions) including comparing tables.

3. Track all DLL and DML changes with possibility to ho back to previous
version.

Hi,

I had similar needs long ago, so I wrote this tool I called Squealer,
which would transform a specification of a SQL database schema into some
PostgreSQL DDL to create a database that implements the same schema in a
logical sense, but actually stores all historical rows, and even permits
existing rows to have references to (soft-)deleted rows, all while
providing modifiable views that simulate the behavior of a regular table as
specified in the input schema through generous use of INSTEAD OF triggers.
It works somewhat like having version control for your database.

You may find the source code here: https://github.com/mgomezch/squealer
Unfortunately, it has hardly any comments, it is completely unmaintained
and probably unused anywhere, I have no idea whether it builds with today's
libraries, and it does not necessarily break the tradeoffs in this space in
a way that fits your use case.

Note there are major caveats with keeping all historical data around
forever, and the semantics of querying historical data can get complicated,
let alone having current data refer to deleted, historical data.  I built
this for a very specific use case where this was the right design, but
please consider very carefully whether this is what you want.

Storing your database history forever would take a lot of space.
Consider whether you can instead keep a record of changes stored outside
the database in some cheap cold storage.  Also consider just keeping a set
of tables with dynamically structured event records (e.g. JSON fields)
partitioned by time ranges and retained only temporarily, perhaps even in a
separate database.  Any such solution will have significant cost and
performance impact if your database bears a nontrivial load, so be careful.

You could also just place your database on a PostgreSQL cluster by itself
and then keep all WAL segments archived forever, so you could just do
point-in-time recovery to any point in the history of your database.  The
space required would grow very quickly, though, so if you don't really need
the full history forever, but only a fixed retention period, you can surely
use any of the well-known solutions for PostgreSQL backups that allow for
this through WAL archiving and periodic basebackups: e.g. WAL-E, WAL-G,

pgBackRest?

If all you need is saving and restoring specific table states, logical
dumps with pg_dump should probably be enough for your needs.

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