Subject: Re: query multiple schemas

From: Dominique Devienne <[email-masked]>
Date: Tue, 23 Apr 2024 09:33:09 +0200

On Sun, Apr 21, 2024 at 11:12 PM Tom Lane <[email-masked]> wrote:

> Steve Baldwin <[email-masked]> writes:
> > If the number of tenant schemas is reasonably static, you could write a
> > plpgsql function to create a set of UNION ALL views
>
> Another idea is to build a partitioned table
>

Hi Norbert. I asked a [similar question][1] a while back,
and unfortunately didn't get any actionable input, perhaps
because I already mentioned in my message the options
proposed here so far. Seems like people like us, using a
dynamic number of schemas, are outliers in database-land.

In my case, the revision/version of the schema could be
different as well, which would complicate the partitioning idea.

In any case, I'm interested in what works well for you.
And if/when I get back to this issue myself, I'll do the same.

Thanks, --DD

[1]: https://postgrespro.com/list/thread-id/2673517

---

From: Norbert Sándor <[email-masked]>
Date: Tue, 23 Apr 2024 11:08:26 +0200

> using a dynamic number of schemas

Although there are historical reasons behind our "schema per tenant"
architecture, it provides very good logical separation of data, and
is very convenient that we don't need to include the tenant ID in
each query (I'm sure that it would cause lots of bugs and trouble).

Besides, we use Hibernate and it
has great support for this architecture.

> In any case, I'm interested in what works well for you.

I went with the dynamic solution I proposed in my original email
(which performs a UNION of the queried table from all tenants).

Performance is currently not a priority in our use-cases, otherwise
I would have probably chosen a more static solution (which wouldn't
be easy to maintain continuously, I'm sure of).

I further simplified its usage, so finally it is fairly comfortable
to use either directly:

select tenantId, (record).*

from tenant_query(null::mytable)

where (record).type=2

order by tenantId, (record).name;

or by using a temporary view:

create temp view
all_tenant_mytable as

select tenantId, (record).* from tenant_query(null::mytable);

select *

from all_tenant_mytable

where type=2

order by tenantId, name;

> In my case, the revision/version of the schema could be
different as well

This complicates things very much, it is probably not possible to
implement a solution as comfortable as in my case.

Although I think the JSON-based solutions proposed in this thread
would work in your case as well.

In my current solution I also use JSON as intermediate
representation - although I feel it is because of my lack
of deeper knowledge of Postgresql's type system.

The difference is that you would need to use JSON as the final
representation, and reference the JSON fields using Postgres's
JSON operators.

> And if/when I get back to this issue myself, I'll do the
same.

My current solution is not much different than the one I posted in
my original question.

My main difficulty was the relatively static nature of Postgresql's
type system, so this solution is a result of lots of trial-and-error
rounds :)

Take a look at it, and you (and maybe others) may have
recommendations, e.g. how I could get rid of the usage of the
intermediate JSON "layer".

(Although I have to admit: it is amazing that it is possible to
implement this at all ;) )

CREATE OR REPLACE FUNCTION tenant_query_json(tbl anyelement)

RETURNS setof json AS $func$

declare

_select text;

begin

_select := (select

string_agg(

format('select t.*, %L tenantId from %I.%I t', schema_name, schema_name, pg_typeof(tbl)),

E'\n' || ' union all ' || E'\n')

from (

SELECT schema_name

FROM information_schema.schemata

where schema_name not in ('information_schema') and schema_name not like 'pg_%'

) tenants

);

return query execute 'select row_to_json(r) from (' || _select || ') as r';

END;

$func$ LANGUAGE plpgsql;

CREATE OR REPLACE FUNCTION tenant_query(tbl anyelement)

RETURNS table(tenantId text, record anyelement) AS $func$

begin

return query

select t.tenantId, t.rec

from (

select

jr->>'tenantid' tenantId,

json_populate_record(tbl, jr) rec

from tenant_query_json(tbl) jr

) t;

END;

$func$ LANGUAGE plpgsql;

As you can see in my examples above, I use the tenant_query() function but in your case
(if your schemas are different) something similar to tenant_query_json() may work better.

--

Norbi

On 2024. 04. 23. 9:33, Dominique
Devienne wrote:

On Sun, Apr 21, 2024 at 11:12 PM Tom Lane <[email-masked]>
wrote:

Steve
Baldwin <[email-masked]>

and unfortunately didn't get any actionable input,
perhaps

different as well, which would complicate the
partitioning idea.

And if/when I get back to this issue myself, I'll do the
same.

Thanks, --DD