Make tuples_per_page pr. table configureable.

Hi.

This is a follow up and updated patch on several old discussions:
http://archives.postgresql.org/pgsql-hackers/2009-07/msg01065.php
http://archives.postgresql.org/pgsql-admin/2010-04/msg00164.php
http://archives.postgresql.org/pgsql-hackers/2009-06/msg00831.php
First patch:
http://archives.postgresql.org/pgsql-hackers/2010-02/msg00096.php

Currently the aim for the amount of tuples per page is 4 resulting
in a target size of the individual tuple to be more than 2KB before
the tupletoaster kicks in. This patch makes it tuneable on a per
table basis.

The main reasoning is that if people have knowledge about the usage
pattern of their database, they can have huge benefit in tuning
TOAST to be more or less aggressive. This is obviously true if:

* The dataset isn't entirely memory cached and
* columns stored in main (and just visibility checking) is more frequently
   done than accessing data columns stored in TOAST.

But even in the case where the dataset is entirely memory cached this
tuneable can transform the database to a widely different performance
numbers than currently. This typically happens in cases where only
visibillity checks are done (select count()) and when aggregates on
stuff stored in main is used.

I must admit that I have chosen a "poor" test data set, since based
on the average length of the tuple the "sweet point" is just around
the current default, but constructing a dataset with an average < 2.5KB
tuple
size would absolutely benefit. But I hope that people can see the benefit
anyway. The dataset is 500.000 records in a table with:

id serial,
code text, (small text block)
entry text (larger text block)

where code is length(code) < 10 and entry:

           avg          | max  | min
-----------------------+------+------
  3640.2042755914488171 | 8708 | 1468

The queries are run multiple time and numbers are based on runs where
iowait was 0 while the query executed. So entirely memory and cpu-bound
numbers:

testdb=# select * from data order by tuples_per_page;
  time_sum_length | time_count | tuples_per_page | main_size | toast_size
-----------------+------------+-----------------+-----------+------------
         5190.258 |     689.34 |               1 | 1981MB    | 0MB
         5478.519 |    660.841 |               2 | 1894MB    | 0MB
         9740.768 |    481.822 |               3 | 1287MB    | 4MB
        12875.479 |     73.895 |(default)      4 | 79MB      | 1226MB
        13082.768 |     58.023 |               8 | 29MB      | 1276MB
(5 rows)

time_sum_length => select sum(length(entry)) from data;
time_count => select count(*) from data;
All timings are in ms.

With this data

Command to set "tuples_per_page" is:
ALTER TABLE <tablename> set (tuples_per_page = X)
where 1 <= X <= 32.

The patch really need some feedback, I've tried to adress Tom Lane's
earlier
comment about fixing the place where it figure out wether it needs a toast
table (and actually tested that it works).

While there surely are more that can be done in order to improve the
flexibillity
in this area I do think that there is sufficient benefit.

This is my second shot at coding C, so please let me know if I have been
doing
anything wrong. Comments are all welcome.

Thanks.

--
Jesper