Horton

Re: [HACKERS] MERGE SQL Statement for PG11

Simon Riggs <simon@2ndquadrant.com>

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From: Simon Riggs <simon@2ndquadrant.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Peter Geoghegan <pg@bowt.ie>, Pavan Deolasee <pavan.deolasee@gmail.com>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, Alvaro Herrera <alvherre@alvh.no-ip.org>, Andrew Dunstan <andrew.dunstan@2ndquadrant.com>, Pavan Deolasee <pavan.deolasee@2ndquadrant.com>, Stephen Frost <sfrost@snowman.net>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2018-03-26T16:16:37Z
Lists: pgsql-hackers 
On 26 March 2018 at 16:09, Robert Haas <robertmhaas@gmail.com> wrote:
> On Mon, Mar 26, 2018 at 5:53 AM, Simon Riggs <simon@2ndquadrant.com> wrote:
>> Since we now have MVCC catalog scans, all the name lookups are
>> performed using the same snapshot so in the above scenario the newly
>> created object would be invisible to the second name lookup.
>
> That's not true, because each lookup would be performed using a new
> snapshot -- not all under one snapshot.

You're saying we take a separate snapshot for each table we lookup?
Sounds weird to me.

So this error could happen in SELECT, UPDATE, DELETE or INSERT as well.

Or you see this as something related specifically to MERGE, if so how?
Please explain what you see.

-- 
Simon Riggs                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Commits

  1. Add support for MERGE SQL command

  2. Avoid repeated name lookups during table and index DDL.

  3. Use an MVCC snapshot, rather than SnapshotNow, for catalog scans.