From: Tom Lane <tgl@sss.pgh.pa.us>
To: "Gopisetty, Ramesh" <rameshg2@illinois.edu>
cc: "pgsql-performance@lists.postgresql.org"
 <pgsql-performance@lists.postgresql.org>
Subject: Re: Performance issue when we use policies for Row Level Security
 along with functions
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Comments: In-reply-to "Gopisetty, Ramesh" <rameshg2@illinois.edu>
	message dated "Wed, 16 Sep 2020 03:39:08 -0000"
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Date: Wed, 16 Sep 2020 10:17:33 -0400
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"Gopisetty, Ramesh" <rameshg2@illinois.edu> writes:
> Policy
> create policy  policy_sel on test FOR SELECT to ram1 USING  (  testkey i=
n (f_sel_policy_test(testkey))  );
> Going to a Sequential scan instead of index scan.  Hence, performance is=
sue.

> If i replace the policy with stright forward without function then it ch=
ooses 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')));

" testkey in ('COMMON',current_setting('ctx_ng'||'.'||'ctx_key_fil')) "
is an indexable condition on testkey, because it compares testkey to
a constant (or at least, a value that's fixed for the life of the query).

" testkey in (f_sel_policy_test(testkey)) "
is not an indexable condition on anything, because there are variables
on both sides of the condition.  So there's no fixed value that the
index can search on.

If you intend f_sel_policy_test() to be equivalent to the other condition,
why are you passing it an argument it doesn't need?

As Luis noted, there's also the problem that an indexable condition
can't be volatile.  I gather that SYS_CONTEXT ends up being a probe
of some GUC setting, which means that marking the function IMMUTABLE
would be a lie, but you ought to be able to mark it STABLE.

			regards, tom lane