Parent/child context relation in pg_get_backend_memory_contexts()

Hi hackers,

pg_get_backend_memory_contexts() (and pg_backend_memory_contexts view)
does not display parent/child relation between contexts reliably.
Current version of this function only shows the name of parent context
for each context. The issue here is that it's not guaranteed that
context names are unique. So, this makes it difficult to find the
correct parent of a context.

How can knowing the correct parent context be useful? One important
use-case can be that it would allow us to sum up all the space used by
a particular context and all other subcontexts which stem from that
context.
Calculating this sum is helpful since currently
(total/used/free)_bytes returned by this function does not include
child contexts. For this reason, only looking into the related row in
pg_backend_memory_contexts does not help us to understand how many
bytes that context is actually taking.

Simplest approach to solve this could be just adding two new fields,
id and parent_id, in pg_get_backend_memory_contexts() and ensuring
each context has a unique id. This way allows us to build a correct
memory context "tree".

Please see the attached patch which introduces those two fields.
Couldn't find an existing unique identifier to use. The patch simply
assigns an id during the execution of
pg_get_backend_memory_contexts() and does not store those id's
anywhere. This means that these id's may be different in each call.

With this change, here's a query to find how much space used by each
context including its children:

> WITH RECURSIVE cte AS (
> SELECT id, total_bytes, id as root, name as root_name
> FROM memory_contexts
> UNION ALL
> SELECT r.id, r.total_bytes, cte.root, cte.root_name
> FROM memory_contexts r
> INNER JOIN cte ON r.parent_id = cte.id
> ),
> memory_contexts AS (
> SELECT * FROM pg_backend_memory_contexts
> )
> SELECT root as id, root_name as name, sum(total_bytes)
> FROM cte
> GROUP BY root, root_name
> ORDER BY sum DESC;

You should see that TopMemoryContext is the one with highest allocated
space since all other contexts are simply created under
TopMemoryContext.

Also; even though having a correct link between parent/child contexts
can be useful to find out many other things as well by only writing
SQL queries, it might require complex recursive queries similar to the
one in case of total_bytes including children. Maybe, we can also
consider adding such frequently used and/or useful information as new
fields in pg_get_backend_memory_contexts() too.

I appreciate any comment/feedback on this.

Thanks,
--
Melih Mutlu
Microsoft