Re: BUG: ReadStream look-ahead exhausts local buffers when effective_io_concurrency>=64

Hi Induja,

On Fri, Feb 27, 2026 at 7:55 PM Induja Sreekanthan <indujas(at)google(dot)com> wrote:
>
> Hi,
>
> I found an issue where Postgres (with effective_io_concurrency of 64 or higher) runs out of local buffers during a sequential scan on a temporary table with TOAST data.
>
> The issue occurs because the ReadStream look-ahead pins all the local buffers. This results in the TOAST index look-up and TOAST page read being unable to find any available local buffers. The ReadStream's max_pinned_buffers can be as high as the num_temp_buffers, depending on the effective_io_concurrency.
>
> Here is a reproduction of the issue using the default temp_buffers setting and effective_io_concurrency=128:
>
> docker run --name my-postgres -e POSTGRES_PASSWORD=my-password -p 5432:5432 -d postgres:18 -c effective_io_concurrency=128
>
> postgres=# CREATE TEMPORARY TABLE tmp_tbl1 (
> s_suppkey NUMERIC NOT NULL,
> s_nationkey NUMERIC,
> s_comment VARCHAR(256),
> s_name CHAR(256),
> s_address VARCHAR(256),
> s_phone TEXT,
> s_acctbal NUMERIC,
> CONSTRAINT supplier_pk PRIMARY KEY (s_suppkey)
> );
> CREATE TABLE
> postgres=# INSERT INTO tmp_tbl1 (s_suppkey, s_nationkey, s_comment, s_name, s_address, s_phone, s_acctbal)
> SELECT
> ('1' || repeat('0', 2000) || i::text)::NUMERIC AS s_suppkey,
> ('5' || repeat('0', 2000) || floor(random() * 25)::text)::NUMERIC AS s_nationkey,
> md5(random()::text) || ' some comment' AS s_comment,
> 'Supplier#' || LPAD(i::text, 9, '0') AS s_name,
> 'Address-' || md5(i::text) AS s_address,
> repeat('P', 4096) || '-' || i::text || repeat('P', 2048) || 'fwoiefrr' ||
> repeat('fejwfelwkmfP', 4096) || '-' || i::text || repeat('fnwekjfmelkwf', 2048) AS s_phone,
> ('9' || repeat('9', 2000) || '.' || floor(random()*100)::text)::NUMERIC AS s_acctbal
> FROM generate_series(1, 8000) AS i;
> INSERT 0 8000
> postgres=# SELECT * FROM tmp_tbl1;
> ERROR: no empty local buffer available

Thanks for reporting this issue. It smells similar to the bug reported
by Alexander earlier. [1] The root cause of them seems the same: we
give read stream too much budget for local buffer pins. The
fix(da6874635db by Melanie) is to cut the budget to 1/4.

> Attached is a patch that addresses this by limiting ReadStream's max_pinned_buffers for temp tables to 75% of the available local buffers. It also introduces a cap on max_ios for temp tables to DEFAULT_EFFECTIVE_IO_CONCURRENCY, to account for multiple sequential scan look-aheads happening simultaneously.

If that's the case, I'm wondering whether it makes sense to backpatch
this fix to 18. I tried to do this for the local tree and the
reproducer passed. That said, it might not be safe to do so for a
stable version. It would be helpful to hear Melanie's and Andres's
thoughts on this.

[1] https://postgr.es/m/97529f5a-ec10-46b1-ab50-4653126c6889%40gmail.com

--
Regards,
Xuneng Zhou
HighGo Software Co., Ltd.