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Re: EXPLAIN: showing ReadStream / prefetch stats

Tomas Vondra <tomas@vondra.me>

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From: Tomas Vondra <tomas@vondra.me>
To: Melanie Plageman <melanieplageman@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2026-03-17T17:51:15Z
Lists: pgsql-hackers

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On 3/16/26 18:49, Tomas Vondra wrote:
> On 3/16/26 18:14, Melanie Plageman wrote:
>> ...
>>
>> I see a couple more issues with the counting in read_stream.c.
>>
>> You are double-counting stalls for synchronous IO. You increment
>> stalls in read_stream_next_buffer() but we actually execute
>> synchronous IO in WaitReadBuffers and return needed_wait as true,
>> which will count a stall again.
>>
>> You are not counting fast path IOs because those don't go through
>> read_stream_start_pending_read() and instead are started directly by
>> StartReadBuffer() in read_stream_next_buffer(). Simple diff attached
>> should fix this.
>>
>> Also, per worker stats are not displayed when BUFFERS false is passed
>> even with IO true because of a small oversight. I fixed it in the
>> attached diff.
>>
> 
> Thanks!
> 

Here's a v2 with your fixes merged. No other changes.


regards

-- 
Tomas Vondra

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. bufmgr: Return whether WaitReadBuffers() needed to wait