/*****************************************************************************

Copyright (c) 2016, 2017, Oracle and/or its affiliates. All Rights Reserved.

This program is free software; you can redistribute it and/or modify it under
the terms of the GNU General Public License, version 2.0, as published by the
Free Software Foundation.

This program is also distributed with certain software (including but not
limited to OpenSSL) that is licensed under separate terms, as designated in a
particular file or component or in included license documentation. The authors
of MySQL hereby grant you an additional permission to link the program and
your derivative works with the separately licensed software that they have
included with MySQL.

This program is distributed in the hope that it will be useful, but WITHOUT
ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
FOR A PARTICULAR PURPOSE. See the GNU General Public License, version 2.0,
for more details.

You should have received a copy of the GNU General Public License along with
this program; if not, write to the Free Software Foundation, Inc.,
51 Franklin St, Fifth Floor, Boston, MA 02110-1301  USA

*****************************************************************************/
Prototype to design the WL#8960 - Partial fetch and update of LOB.

Uncompressed LOB:
================

There are 3 types of pages - data pages, index pages and the first page.  The
data pages contain the LOB data.  The index pages contain the index entries.
The first page is divided into two - the first half contains the index entries
and the second half contains the LOB data.

Each index entry defines one data page.  It contains the following information:

.  the previous index entry (6 bytes)
.  the next index entry (6 bytes)
.  list of older versions (16 bytes)
.  the trx id that created this index entry (6 bytes)
.  the page number (4 bytes)
.  data length. (2 bytes)

Total size of 1 index entry is 40 bytes.

For 16K page size, here are some useful information about the first page:

.  The first page can contain 203 index entries.
.  The first page can contain 8148 bytes of LOB data.
.  The first page is sufficient for an LOB of 3MB size (3317716 bytes).

For 16K page size, here are some useful information about an index page:

.  The index page can contain 408 index entries.
.  The index page can represent 6684672 bytes (6MB) of LOB data.
.  For 1GB of data we need 160 index pages.
.  For 4GB of data we need 640 index pages.

So for a small LOB of about 9MB, we will have 1 first page and 1 index page.

Reading the LOB:
---------------

The header of first page contains the base node of the list of index entries.
Traversing this list will give the latest version of the LOB.  If any index
entry is not to be seen by the reading trx, then its list of older version
is checked to see if it can see any of them.  If no older version is to be
seen, the index entry is skipped.

Modification:
------------

There are 3 operations - replace (size does not change), insert middle at the
given offset (size increases), remove middle at give offset (size decreases).
All the 3 modifications do page level copy-on-write.  If an existing page is
to be modified, then a copy is created and their index entries are adjusted
so that the new version is the latest version and the existing page becomes
an older version.

Rollback and Purge:
-------------------

Given a trxid, the list of modified pages is available from the index
entries.  For rollback and purge operation, the index entries can be
traversed and the relevant index entries can be removed.

Rolling back to savepoint:
--------------------------

Each page contains the FIL_PAGE_LSN. If this is greater than the save point
lsn then remove this page, otherwise retain it.