Re: [GSOC] questions about idea "rewrite pg_dump as library"

On Fri, Apr 12, 2013 at 1:07 PM, Hannu Krosing <hannu(at)2ndquadrant(dot)com>wrote:

> I was just thinking of moving the queries the pg_dump currently
> uses into UDF-s, which do _not_ use catalog cache, but will use
> the same SQL to query catalogs as pg_dump currently does
> using whatever snapshot mode is currently set .
>
> the pg_dump will need to still have the same queries for older
> versions of postgresql but for new versions pg_dump can become
> catalog-agnostic.
>
> and I think that we can retire pg_dump support for older
> postgresql versions the same way we drop support for
> older versions of postgresql itself.

> main things I see would be
>
> * get_list_of_objects(object_**type, pattern or namelist)
> * get_sql_def_for_object(object_**type, object_name)
> * sort_by_dependency(list of [obj_type, obj_name])
>
> from this you could easily construct most uses, especially if
> sort_by_dependency(list of [obj_type, obj_name])
> would be smart enough to break circular dependencies, like
> turning to tables with mutual FK-s into tabledefs without
> FKs + separate constraints.
>
>
+1

This is an excellent idea. This would allow doing all kinds of crazy things
outside of the scope of pg_dump.

2 years ago I was working on a system to version control the schema, inside
the database.
Don't know if it's a good idea or not, but one thing which bugged me a lot
was the lack of pg_get_[object type]def(oid) functions for all different
object types.
It also turned out to be quite complicated to do the pg_depend topological
sort yourself. I managed eventually, but it was running to slow because I
had to pass the entire content of pg_depend to a plperl function I wrote.

With this in place I would be motivated enough to resume my old project,
which is still online at https://github.com/gluefinance/pov if anyone is
interested.

Is it really necessary to write all the missing pg_get_[object
type]def(oid) functions in C? I think it would be quite easy to put them
together using pure SQL, you wouldn't even need PL/pgSQL.

This old view I once wrote manage to produce working create and drop
statements for most object types using SQL only:
https://github.com/gluefinance/pov/blob/master/sql/schema/pov/views/pg_depend_definitions.sql

It would also be nice with functions which returned the proper command to
DROP an object. I need it in this project in order to do schema
modifications where objects have to be dropped/recreated in a particular
order to not break dependencies. Perhaps there are other use cases out
there.