Re: Performance issue when we use policies for Row Level Security along with functions

[luis.roberto@xxxxxxxxxxxxxxx]

De: "Gopisetty, Ramesh" <rameshg2@xxxxxxxxxxxx>
Para: "pgsql-performance" <pgsql-performance@xxxxxxxxxxxxxxxxxxxx>
Enviadas: Quarta-feira, 16 de setembro de 2020 0:39:08
Assunto: Performance issue when we use policies for Row Level Security along with functions

Hi,

I'm seeing a strange behavior when we implement policies (for RLS - Row level security)  using functions. 

table test  consists of columns  testkey,oid,category,type,description... 

Policy

create policy  policy_sel on test FOR SELECT to ram1 USING  (  testkey in (f_sel_policy_test(testkey))  );

Going to a Sequential scan instead of index scan.  Hence, performance issue.

pgwfc01q=> explain analyze select * from test;

                                                 QUERY PLAN

------------------------------------------------------------------------------------------------------------

 Seq Scan on test  (cost=0.00..25713.12 rows=445 width=712) (actual time=1849.592..1849.592 rows=0 loops=1)

   Filter: ((testkey )::text = (f_sel_policy_test(testkey ))::text)

   Rows Removed by Filter: 88930

 Planning Time: 0.414 ms

 Execution Time: 1849.614 ms

(5 rows)

The function is 

CREATE OR REPLACE FUNCTION vpd_sec_usr.f_sel_policy_test(testkey character varying)

RETURNS character varying

LANGUAGE plpgsql

AS $function$

Declare

            v_status character varying;

BEGIN

            if vpd_key = 'COMMON' then

                        return '''COMMON''';

            elsif vpd_key = ('COMMON_' || SYS_CONTEXT('ctx_ng', 'ctx_prod_locale')) then

                        return '''COMMON_' || SYS_CONTEXT('ctx_ng', 'ctx_prod_locale')||'''';

            elsif vpd_key = SYS_CONTEXT('ctx_ng_vpd', 'ctx_key_fil') then

                        return '''co'','''||SYS_CONTEXT('ctx_ng', 'ctx_testkey_fil')||'''';

            end if;

            return 'false';    

            exception when undefined_object then

                        return 'failed';

           

END;

$function$

;

If i replace the policy with stright forward without function then it chooses the index.   Not sure how i can implement with the function.

create policy  policy_sel on test FOR SELECT to ram1 USING  ( testkey in ('COMMON',current_setting('ctx_ng'||'.'||'ctx_key_fil')));

QUERY PLAN

--------------------------------------------------------------------------------------------------------------------

-----------------------------------------------------

 Bitmap Heap Scan on test  (cost=396.66..2966.60 rows=13396 width=712) (actual time=0.693..2.318 rows=13159 loops=1)

   Recheck Cond: ((testkey )::text = ANY ((ARRAY['COMMON'::character varying, (current_setting('ctx_vpd.ctx_key_fil'::text))::character varying])::text[]))

   Heap Blocks: exact=373

   ->  Bitmap Index Scan on test_pkey  (cost=0.00..393.31 rows=13396 width=0) (actual time=0.653..0.653 rows=13159 l

oops=1)

         Index Cond: ((testkey )::text = ANY ((ARRAY['COMMON'::character varying, (current_setting('ctx_vpd.ctx

_key_fil'::text))::character varying])::text[]))

 Planning Time: 0.136 ms

 Execution Time: 2.843 ms

(7 rows)

If i replace the policy with stright forward without function then it chooses the index.   Not sure how i can implement with the function.   I thought of creating the policy with a lot of business logic in the function.  If i have the function then i notice going for full table scan instead of index. 

Please help me if i miss anything in writing a function or how to use functions in the policy. 

Thank you.

Regards,

Ramesh G

---

You could try seeting the function as immutable. By default it is volatile.