WL#6884: PERFORMANCE SCHEMA, USER VARIABLES

In order to get information about the session state, several pieces of
information is needed. One of these pieces of information is knowing if there
are any user-defined variables with values, and what values they have.

The information should be presented in such a way that the output can be fed
into the server to re-establish this part of the session state.

For example, if a result set where the output consists of two columns with
values such that the following code works would be ideal.

for var, val in cursor.fetchall():
   new_conn.cmd_query("SET @`" + var + "` = %s", val)

User Documentation
==================

http://dev.mysql.com/doc/relnotes/mysql/5.7/en/news-5-7-5.html
http://dev.mysql.com/doc/refman/5.7/en/performance-schema-connection-attribute-
tables.html

REQUIREMENTS
============

CURRENT-VERSION := 5.7
PREVIOUS-VERSION := 5.6

Install
-------

Func-Req (1): A fresh MySQL installation of CURRENT-VERSION must create the
following tables:
- 1.1, TABLE performance_schema.user_variables_by_thread

Upgrade
-------

Func-Req (2): An upgrade from PREVIOUS-VERSION to CURRENT-VERSION must create
the following tables:
- 2.1, TABLE performance_schema.user_variables_by_thread

User variables names
--------------------

Func-Req (3): When defining a user variable,
the name given to the user variable:
- (3.1) must have a minimum length of 1
- (3.2) must not end with trailing spaces
- (3.3) must have a maximum length of 64
Setting user variables with illegal names results in an error.

Changes to user variables
=========================

Currently, user variables in the server do not have any limitation on
the length of the variable name.

A new limit for the length of the variable name is enforced,
which matches how identifiers are used in MySQL in general:
an identifier is limited to 64 characters.

Statements such as
  SET @very_long_name_that_exceeds_64_characters = ...
now fail with an error.

TABLE performance_schema.user_variables_by_thread
=================================================

New table

This table exposes user variables associated with each thread.

Columns

  `THREAD_ID` bigint(20) unsigned NOT NULL,
  `VARIABLE_NAME` varchar(64) NOT NULL,
  `VARIABLE_VALUE` longblob

Note:
"BLOB" are limited to 16 bits,
using a "LONGBLOB" here, to store up to 32 bits values (4 Gb).
The real limit on user variables size may be lower,
due to constraints related to max_allowed_packet.

Operations:

Only SELECT is supported, this is a read only table.

Semantic for each columns

When a user variable in thread TID is defined with
set @foo = bar
a row exposes the variable as follows:
- THREAD_ID = TID
- VARIABLE_NAME = foo
- VARIABLE_VALUE = bar