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From: Nathan Bossart <[email protected]>
To: Tom Lane <[email protected]>
Cc: [email protected]
Cc: alex work <[email protected]>
Cc: Robert Haas <[email protected]>
Subject: Re: Slow GRANT ROLE on PostgreSQL 16 with thousands of ROLEs
Date: Thu, 21 Mar 2024 15:40:12 -0500
Message-ID: <20240321204012.GA1992418@nathanxps13> (raw)
In-Reply-To: <[email protected]>
References: <CAGvXd3OSMbJQwOSc-Tq-Ro1CAz=vggErdSG7pv2s6vmmTOLJSg@mail.gmail.com>
	<[email protected]>
	<[email protected]>
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On Thu, Mar 21, 2024 at 04:31:45PM -0400, Tom Lane wrote:
> I wrote:
>> ... I still see the problematic GRANT taking ~250ms, compared
>> to 5ms in v15.  roles_is_member_of is clearly on the hook for that.
> 
> Ah: looks like that is mainly the fault of the list_append_unique_oid
> calls in roles_is_member_of.  That's also an O(N^2) cost of course,
> though with a much smaller constant factor.
> 
> I don't think we have any really cheap way to de-duplicate the role
> OIDs, especially seeing that it has to be done on-the-fly within the
> collection loop, and the order of roles_list is at least potentially
> interesting.  Not sure how to make further progress without a lot of
> work.

Assuming these are larger lists, this might benefit from optimizations
involving SIMD intrinsics.  I looked into that a while ago [0], but the
effort was abandoned because we didn't have any concrete use-cases at the
time.  (I'm looking into some additional optimizations in a separate thread
[1] that would likely apply here, too.)

[0] https://postgr.es/m/20230308002502.GA3378449%40nathanxps13
[1] https://postgr.es/m/20240321183823.GA1800896%40nathanxps13

-- 
Nathan Bossart
Amazon Web Services: https://aws.amazon.com

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