Re: [PERFORM] Row level security policy policy versus SQL constraints. Any performance difference?

[Joe Carlson]

Thanks for your suggestions.

I had pretty much given up on this idea. At first, I had thought there 
would only be 2 or 3 different constraint cases to consider. I had 
thought of using distinct credentials for my connection and using RLS to 
give different cuts on the same table. The different policies could be 
established in advance and never touched.


But then it became clear that I actually would need a very large number 
of different restrictions on the tables - too many to create in advance. 
At this point it's easiest to apply constraints on each select rather 
than apply a policy every time.


Thanks,

Joe

On 10/17/2017 03:06 PM, Tom Lane wrote:

Tomas Vondra  writes:

On 10/17/2017 10:44 PM, Joe Carlson wrote:

What I was wondering is what is the performance differences between a
row level security implementation:
...
and an implementation where I add on the constraints as part of each
select statement:

The main point of the RLS is enforcing an order in which the conditions
are evaluated.

Yeah.  Because of that, I would *not* recommend RLS if you can equally
well stick the equivalent conditions into your queries.  There is way
too much risk of taking a serious performance hit due to a bad plan.

An alternative you might consider, if simplifying the input queries
is useful, is to put the fixed conditions into a view and query the
view instead.  That way there's not an enforced evaluation order.

regards, tom lane




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Re: [PERFORM] Row level security policy policy versus SQL constraints. Any performance difference?

[Tom Lane]

Tomas Vondra  writes:
> On 10/17/2017 10:44 PM, Joe Carlson wrote:
>> What I was wondering is what is the performance differences between a
>> row level security implementation:
>> ...
>> and an implementation where I add on the constraints as part of each
>> select statement:

> The main point of the RLS is enforcing an order in which the conditions
> are evaluated.

Yeah.  Because of that, I would *not* recommend RLS if you can equally
well stick the equivalent conditions into your queries.  There is way
too much risk of taking a serious performance hit due to a bad plan.

An alternative you might consider, if simplifying the input queries
is useful, is to put the fixed conditions into a view and query the
view instead.  That way there's not an enforced evaluation order.

regards, tom lane


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Re: [PERFORM] Row level security policy policy versus SQL constraints. Any performance difference?

[Tomas Vondra]

Hi,

On 10/17/2017 10:44 PM, Joe Carlson wrote:
> Hello.
> 
> I have not used row level security policies in the past but am
> considering using them for a project in which I would like to restrict
> the set returned in a query based on specific fields. This is more as a
> convenience issue (for me) rather than a security issue.
> 
> What I was wondering is what is the performance differences between a
> row level security implementation:
> 
> CREATE POLICY  ON  TO  USING
> (=ANY());
> 
> DROP POLICY 
> 
> and an implementation where I add on the constraints as part of each
> select statement:
> 
> SELECT  FROM  WHERE  AND
> =ANY()
> 
> In my (admittedly small) number of EXPLAINs I've looked at, it appears
> that the policy logic is added to the SELECT statement as a constraint.
> So I would not expect any fundamental performance difference in the 2
> different forms.
> 
> Is this true? Or is there some extra behind-the-scenes things to be
> aware of? Can there be excessive overhead from the CREATE/DROP POLICY
> statements?
> 

The main point of the RLS is enforcing an order in which the conditions
are evaluated. That is, the "security" quals (coming from RLS policies)
have to be evaluated first, before any quals that might leak information
about the values (imagine a custom PL/pgSQL function inserting the data
somewhere, or perhaps just printing debug messages).

(Many built-in operators are however exempt from that, as we consider
them leak-proof. This allows us to use non-RLS conditions for index
scans etc. which might be impossible otherwise)

Otherwise yes - it's pretty much the same as if you combine the
conditions using AND. It's "just" much more convenient approach.

regards

-- 
Tomas Vondra  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services


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[PERFORM] Row level security policy policy versus SQL constraints. Any performance difference?

[Joe Carlson]

Hello.

I have not used row level security policies in the past but am 
considering using them for a project in which I would like to restrict 
the set returned in a query based on specific fields. This is more as a 
convenience issue (for me) rather than a security issue.


What I was wondering is what is the performance differences between a 
row level security implementation:


CREATE POLICY  ON  TO  USING 
(=ANY());


DROP POLICY 

and an implementation where I add on the constraints as part of each 
select statement:


SELECT  FROM  WHERE  AND =ANY()

In my (admittedly small) number of EXPLAINs I've looked at, it appears 
that the policy logic is added to the SELECT statement as a constraint. 
So I would not expect any fundamental performance difference in the 2 
different forms.


Is this true? Or is there some extra behind-the-scenes things to be 
aware of? Can there be excessive overhead from the CREATE/DROP POLICY 
statements?


Thanks,

Joe


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