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From: Christophe Pettus <[email protected]>
To: Eden Aharoni <[email protected]>
Cc: [email protected] <[email protected]>
Subject: Re: RDS IO Read time
Date: Mon, 31 Mar 2025 08:15:39 -0700
Message-ID: <[email protected]> (raw)
In-Reply-To: <DBBPR03MB102227BDDC40A11527BF6C0C0BBAD2@DBBPR03MB10222.eurprd03.prod.outlook.com>
References: <DBBPR03MB102227BDDC40A11527BF6C0C0BBAD2@DBBPR03MB10222.eurprd03.prod.outlook.com>



> On Mar 31, 2025, at 06:54, Eden Aharoni <[email protected]> wrote:
> Is this expected IO read rate? I can’t help but feel we’re missing something here..

Really, no particular I/O rate is "expected": if PostgreSQL needs that much data, it'll use that much I/O to get it.  From your description, it's likely that it's a case of the working set for the database just not fitting into the memory you have, so PostgreSQL needs to go out to secondary storage a lot to fetch the data.

The best first step is to use Performance Insights to see which queries are using I/O, and run sample ones with EXPLAIN (ANALYZE, BUFFERS) to see where the I/O is being used within the query.  Given that you allow users to assemble arbitrary queries, it's likely that PostgreSQL is having to use a wide variety of indexes (or sequential scans), so it can't successfully cache a particular set in memory.

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