[pgsql-GENERAL] AUTOVACUUM and Streaming Replication in v9.5

Hi,

I was wondering if there is some change in the way
max_standby_streaming_delay and hot_standby_feedback work in v9.5.

Below is a scenario which I had built to test out something but the
behavior confused me. I would like to check if there is something/some
parameter which I am missing or if my understanding is wrong.

I have a Standby Server with hot_standby_feedback set to off.

My max_standby_streaming_delay is set to 30s.

I started a transaction on standby with repeatable read isolation level. I
fired a select

select count(*) from table_a;

This gave me result 60000;

I deleted a huge chunk of rows on Primary for table_a.

I can see that autovaccum lauched an autovacuum and autoanalyze (going by
the timestamp in pg_stat_user_tables for table_a) after these deletes. For
a short period of time I also saw that pg_class.reltuples was reduced to a
smaller number (I think because of deletions).

But when I checked again, pg_stat_user_tables on master revealed that
autovacuum has triggered a few times since the deletion for table_a. I also
noted that on master pg_class.reltuples are back to roughly 60000 (which
was the case before deletion).

I thought if my hot_standby_feedback is off, I might face an issue with
rows being removed by vacuum. But that did not happen. Can someone help me
to understand how MVCC, autovacuum work with Streaming replication.

Regards
Sameer
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Best Regards,

*Sameer Kumar | DB Solution Architect*

*ASHNIK PTE. LTD.*

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