pageinspect patch, for showing tuple data

Hi!

I've created a patch for pageinspect that allows to see data stored in the
tuple.

This patch has two main purposes:

1. Practical: Make manual DB recovery more simple
2. Educational: Seeing what data is actually stored in tuple, allows to get
better understanding of how does postgres actually works.

This patch adds several new functions, available from SQL queries. All these
functions are based on heap_page_items, but accept slightly different
arguments and has one additional column at the result set:

heap_page_tuples - accepts relation name, and bulkno, and returns usual
heap_page_items set with additional column that contain snapshot of tuple data
area stored in bytea.

heap_page_tuples_attributes - same as heap_page_tuples, but instead of single
tuple data bytea snapshot, it has array of bytea values, that were splitted
into attributes as they would be spitted by nocachegetattr function (I
actually reimplemented this function main algorithm to get this done)

heap_page_tuples_attrs_detoasted - same as heap_page_tuples_attrs, but all
varlen attributes values that were compressed or TOASTed, are replaced with
unTOASTed and uncompressed values.

There is also one strange function: _heap_page_items it is useless for
practical purposes. As heap_page_items it accepts page data as bytea, but also
get a relation name. It tries to apply tuple descriptor of that relation to
that page data.
This would allow you to try to read page data from one table using tuple
descriptor from anther. A strange idea, one should say. But this will allow
you: a) See how wrong data can be interpreted (educational purpose).
b) I have plenty of sanity check while reading parsing that tuple, for this
function I've changed error level from ERROR to WARNING. This function will
allow to write proper tests that all these checks work as they were designed
(I hope to write these tests sooner or later)

I've also added raw tuple data output to original heap_page_items function,
thought I am not sure if it is good idea. I just can add it there so I did it.
May be it would be better to change it back for better backward compatibility.

Attached patched is in "almost ready" state. It has some formatting issues.
I'd like to hear HACKER's opinion before finishing it and sending to
commitfest.

--
Nikolay Shaplov
Postgres Professional: http://www.postgrespro.com
Russian Postgres Company