Обсуждение: [PERFORM] Row level security policy policy versus SQL constraints. Anyperformance difference?
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 <policy name> ON <table> TO <role> USING (<field>=ANY(<values>)); <series of selects> DROP POLICY <policy name> and an implementation where I add on the constraints as part of each select statement: SELECT <whatever> FROM <table> WHERE <constraints> AND <field>=ANY(<values>) 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 -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
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 <policy name> ON <table> TO <role> USING > (<field>=ANY(<values>)); > <series of selects> > DROP POLICY <policy name> > > and an implementation where I add on the constraints as part of each > select statement: > > SELECT <whatever> FROM <table> WHERE <constraints> AND > <field>=ANY(<values>) > > 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 -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Tomas Vondra <tomas.vondra@2ndquadrant.com> 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
--
Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-performance
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 <tomas.vondra@2ndquadrant.com> 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 -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance