WL#13588: Deprecate support for prefix keys in partition functions

When testing MySQL support for prefix keys in partition functions, we see that
the implementation does not actually take into account the prefix key (!).

When a table includes a column having prefix key index in
the PARTITION BY KEY clause, that column has no effect on
partition calculations.

eg: CREATE TABLE `t1` (
        `a` VARCHAR(10000), 
        `b` VARCHAR(25), 
        `c` VARCHAR(10), 
        PRIMARY KEY (a(10), b, c(2))
    ) PARTITION BY KEY() PARTITIONS 2;

    In this example, the columns `a` and `c` are not
    considered during partition calculations.


Implementing this functionality now in 8.0 will result in different partitioning
in existing tables, and hence requiring the tables to be re-partitioned, with
added challenges for cross-version replication. 
(This behavior is seen for all SEs supporting partitioning. Additionally, NDB 
does not support index prefixes altogether).

The functionality is therefore suggested deprecated in 8.0.

This functionality is not widely used, so we consider
deprecation and removal to affect few use-cases.

Functional requirements:
````````````````````````

 FR1: On CREATE TABLE, if the table has prefix key indexes
 in the PARTITION BY KEY() function, a deprecation warning
 shall be sent to the client for each prefix key index used.

 FR2: On ALTER TABLE, if the table has prefix key indexes in
 the PARTITION BY KEY() function, a deprecation warning
 shall be sent to the client for each prefix key index used.

Upgrade Related:
----------------

 U1) On upgrade from 5.7, if an existing table has prefix
 key indexes in the PARTITION BY KEY() function, the
 deprecation warning will be printed in the error log for
 each prefix key index used.

 U2) On upgrade from lower 8.0.x version, if an existing
 table has prefix key indexes in the PARTITION BY KEY()
 function, the deprecation warning will be printed in the
 error log for each prefix key index used.

I. INTRODUCTION:
````````````````
 This WL aims to throw deprecation warnings when a table
 uses prefix key indexes in the PARTITION BY KEY() function.
 The below sections provide more detail.


II. DESIGN:
```````````

 1. Changes to current behaviour:
 --------------------------------

 On running CREATE TABLE or ALTER TABLE, if a prefix key
 index is detected in the PARTITION BY KEY() function,
 deprecation warnings will be sent to the client, specifying
 each prefix key index used.

 Note: The ALTER TABLE query being run on an affected table
 need not have anything to do with partitioning, eg: even
 adding a new column would trigger the warning.

 2. Affected Tables:
 -------------------

 Tables having definition as given below will be affected.

 CREATE TABLE tbl_name (
      prefix_col {CHAR|VARCHAR|BINARY|VARBINARY} (length), 
      ...
      {INDEX|PRIMARY KEY} (prefix_col(index_length), ...)
 ) PARTITION BY 
      [LINEAR] KEY [ALGORITHM={1|2}] () 
   PARTITIONS num;

 
 3. Why only CHAR|VARCHAR|BINARY|VARBINARY datatypes?
 ----------------------------------------------------
 According to documentation, prefixes CAN be specified for
 CHAR, VARCHAR, BINARY, and VARBINARY columns in index
 definitions.
 
 Additionally, a prefix MUST be specified for BLOB and TEXT
 columns in index definitions.
 
 However, when partitioning by [LINEAR] KEY, it is NOT
 possible to use columns of TEXT or BLOB as partitioning
 keys.

 This means that only CHAR, VARCHAR, BINARY, and VARBINARY
 columns may be used as both prefix indexes and in the
 PARTITION BY KEY() function.


 4) What happens when prefix length = column length?
 ---------------------------------------------------
 When the prefix length is the same as column length,
 it is equivalent to not specifying a prefix index at all,
 and would not trigger the deprecation warning. 
 
  eg: CREATE TABLE `t1` (
           `a` VARCHAR(100), 
           `b` VARCHAR(25), 
           PRIMARY KEY (a(100), b)
       ) PARTITION BY KEY() PARTITIONS 2;
      
      is equivalent to 

      CREATE TABLE `t1` (
           `a` VARCHAR(100), 
           `b` VARCHAR(25), 
           PRIMARY KEY (`a`,`b`)
       ) PARTITION BY KEY() PARTITIONS 2;


III. UPGRADE:
`````````````
 On upgrade, deprecation warnings will be sent to the error
 log, specifying each prefix key index used in the PARTITION
 BY KEY() function.

 This applies to both upgrade from 5.7 as well as upgrade
 from lower 8.0.x version.


IV. PERFORMANCE:
````````````````
 There shouldn't be any performance issues caused by this
 WL's changes.


V. DOCUMENTATION:
`````````````````
 Update to documentation is required specifying that the use
 of prefix indexes in the PARTITION BY KEY() function has no
 effect on partition calculations, and is deprecated.