Request for new column in pg_namespace

Hi

ne 15. 12. 2024 v 17:59 odesílatel Ron Johnson <ronljohnsonjr@gmail.com>
napsal:

https://www.postgresql.org/docs/current/catalog-pg-namespace.html

Currently, when I want to query all "userland" tables, I write something
like:
select ...
from pg_class cl, pg_namespace nsp
where cl.relnamespace = nsp.oid
and nsp.nspname not like 'pg_%
and nsp.nspname != 'information_schema';

A new boolean column named "indissystem" that's true only for system
relations would make *many* maintenance queries cleaner, since they'd
look like:
select ...
from pg_class cl, pg_namespace nsp
where cl.relnamespace = nsp.oid
and nsp.indissystem = false;

oid of all system objects is less then 0x4000

Regards

Pavel

Pavel Stehule <pavel.stehule@gmail.com> writes:

ne 15. 12. 2024 v 17:59 odesílatel Ron Johnson <ronljohnsonjr@gmail.com>
napsal:

A new boolean column named "indissystem" that's true only for system
relations would make *many* maintenance queries cleaner, since they'd
look like:
select ...

oid of all system objects is less then 0x4000

That wouldn't help for excluding temp schemas, and it's not totally
trustworthy for information_schema either.

But I think the real problem with Ron's proposal is that it presumes
there is a one-size-fits-all notion of "system schema". As a
counterexample, for some maintenance activities (such as vacuuming)
you might wish to process pg_catalog.

What I'd suggest as an improvement that could be implemented
immediately is to wrap the checks in a user-defined function
like "is_system_schema(nspname name)".

regards, tom lane

On Sun, 15 Dec 2024 at 12:29, Tom Lane <tgl@sss.pgh.pa.us> wrote:

What I'd suggest as an improvement that could be implemented
immediately is to wrap the checks in a user-defined function
like "is_system_schema(nspname name)".

Would it make sense to make the parameter be of type regnamespace?

On Sun, Dec 15, 2024 at 12:29 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:

Pavel Stehule <pavel.stehule@gmail.com> writes:

ne 15. 12. 2024 v 17:59 odesílatel Ron Johnson <ronljohnsonjr@gmail.com>
napsal:

A new boolean column named "indissystem" that's true only for system
relations would make *many* maintenance queries cleaner, since they'd
look like:
select ...

oid of all system objects is less then 0x4000

That wouldn't help for excluding temp schemas, and it's not totally
trustworthy for information_schema either.

But I think the real problem with Ron's proposal is that it presumes
there is a one-size-fits-all notion of "system schema". As a
counterexample, for some maintenance activities (such as vacuuming)
you might wish to process pg_catalog.

In that case, one would explicitly mention pg_catalog, no?
where cl.relnamespace = nsp.oid
and (nsp.indissystem = false or nsp.nspname = 'pg_catalog');

What I'd suggest as an improvement that could be implemented
immediately is to wrap the checks in a user-defined function
like "is_system_schema(nspname name)".

Good idea.

--
Death to <Redacted>, and butter sauce.
Don't boil me, I'm still alive.
<Redacted> lobster!

Isaac Morland <isaac.morland@gmail.com> writes:

On Sun, 15 Dec 2024 at 12:29, Tom Lane <tgl@sss.pgh.pa.us> wrote:

What I'd suggest as an improvement that could be implemented
immediately is to wrap the checks in a user-defined function
like "is_system_schema(nspname name)".

Would it make sense to make the parameter be of type regnamespace?

Meh ... you could, but what the function really needs is the name.
Getting from regnamespace (which is an OID) to the name would incur
an extra syscache lookup. Admittedly, if it removes the need for
the calling query to join to pg_namespace at all, you'd probably
come out about even --- the net effect would be about like a
hashjoin to pg_namespace, I think, since the syscache would act
like the inner hashtable of a hashjoin.

regards, tom lane

On Sun, 15 Dec 2024 at 14:20, Tom Lane <tgl@sss.pgh.pa.us> wrote:

Isaac Morland <isaac.morland@gmail.com> writes:

On Sun, 15 Dec 2024 at 12:29, Tom Lane <tgl@sss.pgh.pa.us> wrote:

What I'd suggest as an improvement that could be implemented
immediately is to wrap the checks in a user-defined function
like "is_system_schema(nspname name)".

Would it make sense to make the parameter be of type regnamespace?

Meh ... you could, but what the function really needs is the name.
Getting from regnamespace (which is an OID) to the name would incur
an extra syscache lookup. Admittedly, if it removes the need for
the calling query to join to pg_namespace at all, you'd probably
come out about even --- the net effect would be about like a
hashjoin to pg_namespace, I think, since the syscache would act
like the inner hashtable of a hashjoin.

Thanks for the critique. It occurs to me that this function is perhaps just
as much about “is this name a system-reserved name?” as “is this schema a
system schema?”. So putting in a name that doesn’t actually exist in the
database should be perfectly valid, which of course only works if it takes
a string. Generally speaking I am a big fan of the reg* data types but in
this specific case I think it’s not a clear win. I might still suggest
providing both versions using function overloading.

On Sun, Dec 15, 2024 at 2:20 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:

Isaac Morland <isaac.morland@gmail.com> writes:

On Sun, 15 Dec 2024 at 12:29, Tom Lane <tgl@sss.pgh.pa.us> wrote:

What I'd suggest as an improvement that could be implemented
immediately is to wrap the checks in a user-defined function
like "is_system_schema(nspname name)".

Would it make sense to make the parameter be of type regnamespace?

Meh ... you could, but what the function really needs is the name.
Getting from regnamespace (which is an OID) to the name would incur
an extra syscache lookup. Admittedly, if it removes the need for
the calling query to join to pg_namespace at all, you'd probably
come out about even --- the net effect would be about like a
hashjoin to pg_namespace, I think, since the syscache would act
like the inner hashtable of a hashjoin.

It'll simplify the SQL to pass it a pg_class.relnamespace value, since
that's what's stored in pg_class.

select ...
from pg_class cl INNER JOIN ...
where not is_system_schema(cl.relnamespace)
and ...;

Might it be slightly slower? Sure... but pg_class and pg_namespace aren't
giant tables, and the queries won't run thousands of times per day. Thus,
in this case, a little less efficiency for much cleaner code is an
acceptable trade-off TO ME.

Heck, given how often "pg_class cl INNER JOIN pg_namespace nsp ON
cl.relnamespace = nsp.oid" appears in my (and so much other code around the
Internet), I should probably create a view that joins the two tables, and
adds an is_system_schema column.

That would *really* simplify my code...

--
Death to <Redacted>, and butter sauce.
Don't boil me, I'm still alive.
<Redacted> lobster!