Horton

Re: Libpq support to connect to standby server as priority

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Laurenz Albe <laurenz.albe@cybertec.at>, Haribabu Kommi <kommi.haribabu@gmail.com>, Jing Wang <jingwangian@gmail.com>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>, "Tsunakawa, Takayuki" <tsunakawa.takay@jp.fujitsu.com>
Date: 2018-11-21T14:05:08Z
Lists: pgsql-hackers 
On Fri, Nov 16, 2018 at 11:35 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Oh!  The reason I assumed it wasn't doing that is that such a behavior
> seems completely insane.  If the point is to keep down the load on your
> master server, then connecting only to immediately disconnect is not
> a friendly way to do that --- even without counting the fact that you
> might later come back and connect again.

That seems like a really weak argument.  Opening a connection to the
master surely isn't free, but it must be vastly cheaper than the cost
of the queries you intend to run.  I mean, no reasonable production
user of PostgreSQL opens a connection, runs one or two short queries,
and then closes the connection.  You open a connection and keep it
open for minutes, hours, days, or longer, running hundreds, thousands,
or millions of queries.  The cost of checking whether you've got a
master or a standby is a drop in the bucket.

And, I mean, if there's some scenario where what I just said isn't
true, well then don't use this feature in that particular case.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Commits

  1. Make test_target_session_attrs more robust against connection failure.

  2. Extend the abilities of libpq's target_session_attrs parameter.

  3. Mark default_transaction_read_only as GUC_REPORT.

  4. Introduce a new GUC_REPORT setting "in_hot_standby".

  5. Avoid spamming the client with multiple ParameterStatus messages.

  6. Avoid harmless Valgrind no-buffer-pin errors.

  7. Restructure libpq code to remove some duplicity

  8. libpq: Add target_session_attrs parameter.