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From: Ayush Vatsa <[email protected]>
To: Tom Lane <[email protected]>
Cc: David G. Johnston <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Subject: Re: Clarification on Role Access Rights to Table Indexes
Date: Tue, 18 Feb 2025 01:12:44 +0530
Message-ID: <CACX+KaMiZaFWVxYzZ_Lw-EBKgiO5GEBHmHREqs=GDpM88hRqdw@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>
References: <CACX+KaMz2ZoOojh0nQ6QNBYx8Ak1Dkoko=D4FSb80BYW+o8CHQ@mail.gmail.com>
	<[email protected]>
	<CAKFQuwZ+EsCJHmBVdHeJ2XUWUBSGtN8k2icrX2hrPR=m7sLNGg@mail.gmail.com>
	<[email protected]>

> As it stands, a superuser can prewarm an index (because she bypasses all
> privilege checks including this one), but nobody else can.
That's not fully true. Any role can prewarm an index if the role has the
correct privileges.
postgres=# GRANT CREATE ON SCHEMA PUBLIC TO alpha;
GRANT
postgres=# SET ROLE alpha;
SET
postgres=> CREATE TABLE tab(id INT);
CREATE TABLE
postgres=> CREATE INDEX tab_idx ON tab(id);
CREATE INDEX
postgres=> SELECT pg_prewarm('tab_idx');
 pg_prewarm
------------
          1
(1 row)

Don't know what stopped it from prewarming the catalog table index.
Maybe it's checking who is the owner of the table. Although in code
it's just checking the ACL_SELECT [1]. I will be debugging more here
and maybe create a patch for the same.

postgres=# RESET ROLE;
RESET
postgres=# CREATE TABLE superuser_tab(id INT);
CREATE TABLE
postgres=# CREATE INDEX idx_superuser_tab ON superuser_tab(id);
CREATE INDEX
postgres=# GRANT SELECT ON superuser_tab TO alpha;
GRANT
postgres=# SET ROLE alpha;
SET
postgres=> SELECT pg_prewarm('superuser_tab');
 pg_prewarm
------------
          0
(1 row)
postgres=> SELECT pg_prewarm('idx_superuser_tab');
ERROR:  permission denied for index idx_superuser_tab

postgres=> RESET ROLE;
RESET
postgres=# ALTER TABLE superuser_tab OWNER TO alpha;
ALTER TABLE
postgres=# SET ROLE alpha;
SET
postgres=> SELECT pg_prewarm('idx_superuser_tab');
 pg_prewarm
------------
          1
(1 row)

But I agree we should just check the table privileges even in the case of
indexes to decide whether to prewarm or not, as
*indexes don't have any privilegesof their own.*

> I think -hackers would be the appropriate location for that.
I am shifting this to -hackers mailing list instead of general.

[1]
https://github.com/postgres/postgres/blob/master/contrib/pg_prewarm/pg_prewarm.c#L108-L110

Regards,
Ayush Vatsa
SDE AWS

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  Subject: Re: Clarification on Role Access Rights to Table Indexes
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