Backport the ->> operator to MySQL 5.7

[Norvald H. Ryeng]

As agreed with Robie Basak (racb), I'm filing this request for
backporting a feature from 5.8 to the 5.7 stable branch so that it can
be considered by the SRU Team and returned as input to
upstream. Upstream is waiting for response from distros before
deciding whether to backport or not.

Background
==========

MySQL 5.7 introduced a new JSON datatype, functions and operators to
inspect and modify JSON documents. The JSON support has received a lot
of attention and user feedback, and after requests from users, a new
operator was introduced in MySQL 5.8 that makes the syntax more
readable and easy to use.

The new operator is only syntactic sugar, not new functionality.

The new operator "->>" is a shorthand that allows the user to write

    json_field->>"$.a_key"

instead of

    JSON_UNQUOTE(JSON_EXTRACT(json_field, "$.a_key"))

Because of the immediate user requests on a new feature, upstream
would like to backport this syntax to MySQL 5.7. If backported, it
will be done after the release of Ubuntu 16.04, so it will affect an
existing Ubuntu release.

This does not fall under the existing microrelease exception for
MySQL, and so must be considered separately. In consideration of the
microrelease exception requirements, upstream is interested in a
decision for acceptance in an Ubuntu SRU before making the change
upstream, so that any change made, or not made, is consistent across
upstream, Ubuntu and any other distribution that wish to follow suit.

We would like a +1 or a -1 from the SRU team (or Technical Board if
appropriate) on backporting it to 5.7.

Impact
======

If backported, the new syntax will be a built-in, non-optional part of
the server. It is not suited for implementation in an optional plugin.

MySQL 5.7 is not yet in Ubuntu, but will be when the new syntax can be
backported.

JSON functionality is new in 5.7, so the user base is limited. No
packages in Ubuntu currently use this functionality.

Regression potential
====================

The potential for regression is considered low for the following
reasons:

 - The new operator is pure syntactic sugar, no new functionality is
   added

 - The change is very small and localized

 - The change allows new syntax, no existing syntax is affected

 - No packages in Ubuntu currently use the JSON functionality of MySQL

[Steve Langasek (vorlon)]

Thank you for the thorough SRU analysis. I agree that this does not fit the standing MRE. I also agree, from your description of the feature, that this poses minimal risk to an SRU. For the SRU team, I'm +0 on whether this should be backported to mysql 5.7 - but I'm +1 on accepting this as part of an SRU if upstream chooses to backport it.

[Norvald H. Ryeng (nryeng)]

This feature has been backported to MySQL 5.7.13.

[Nish Aravamudan (nacc)]

5.7.13-0ubuntu3 is in 16.10.

[Robie Basak (racb)]

5.7.15-0ubuntu0.16.04.1 is in xenial-updates.