Horton

Re: Replication slot stats misgivings

Masahiko Sawada <sawada.mshk@gmail.com>

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From: Masahiko Sawada <sawada.mshk@gmail.com>
To: Andres Freund <andres@anarazel.de>
Cc: Amit Kapila <amit.kapila16@gmail.com>, PostgreSQL-development <pgsql-hackers@postgresql.org>, Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>
Date: 2021-04-01T12:48:44Z
Lists: pgsql-hackers 
On Tue, Mar 30, 2021 at 9:58 AM Andres Freund <andres@anarazel.de> wrote:
>
> IMO, independent of the shutdown / startup issue, it'd be worth writing
> a patch tracking the bytes sent independently of the slot stats storage
> issues. That would also make the testing for the above cheaper...

Agreed.

I think the bytes sent should be recorded by the decoding plugin, not
by the core side. Given that table filtering and row filtering,
tracking the bytes passed to the decoding plugin would not help gauge
the actual network I/O. In that sense, the description of stream_bytes
in the doc seems not accurate:

---
This and other streaming counters for this slot can be used to gauge
the network I/O which occurred during logical decoding and allow
tuning logical_decoding_work_mem.
---

It can surely be used to allow tuning logical_decoding_work_mem but it
could not be true for gauging the network I/O which occurred during
logical decoding.

Regards,

-- 
Masahiko Sawada
EDB:  https://www.enterprisedb.com/

Commits

  1. Doc: Update logical decoding stats information.

  2. Fix tests for replication slots stats.

  3. Update replication statistics after every stream/spill.

  4. Fix the computation of slot stats for 'total_bytes'.

  5. Another try to fix the test case added by commit f5fc2f5b23.

  6. Use HTAB for replication slot statistics.

  7. Fix test case added by commit f5fc2f5b23.

  8. Add information of total data processed to replication slot stats.

  9. Use NameData datatype for slotname in stats.