GIN improvements

Improvements of GIN indexes were presented on PGCon 2008. Presentation:
  http://www.sigaev.ru/gin/fastinsert_and_multicolumn_GIN.pdf

1) multicolumn GIN
This patch ( http://www.sigaev.ru/misc/multicolumn_gin-0.2.gz ) adds multicolumn
support to GIN. The basic idea is: keys (entries in GIN terminology) extracted
from values are stored in separated tuples along with their column number. In
that case, multicolumn clause is  just AND of column's clauses. Unlike other
indexes, the performance of search doesn't depends on what column of index
(first, last, any subset) is used in search clause. This property can be used in
gincostestimate, but I haven't looked on it yet.

2) fast insert into GIN
This patch ( http://www.sigaev.ru/misc/fast_insert_gin-0.4.gz ) implements an
idea of using bulk insert technique, which used at index creation time. Inserted
rows are stored in the linked list of pending pages and inserted to the regular
structure of GIN at vacuum time. The algorithm is shown in presentation, but
insert completion process (vacuum) was significantly reworkes to improve
concurrency. Now, the list of pending page is locked much lesser time - only
during deletion of pages from the list.

Open item:
what is a right time to call insert completion? Currently, it is called by
ginbulkdelete and ginvacuumcleanup, ginvacuumcleanup will call completion if
ginbulkdelete wasn't called. That's not good, but works. Completion process
should started before ginbulkdelete because ginbulkdelete doesn't look on
pending pages at all.

Since insert completion (of any index if that method will exists, I think) runs
fast if number of inserted tuples is a small because it doesn't go through the
whole index, so, IMHO, the existing statistic's fields should not be changed.
That idea, discussed at PGCon, is to have trigger in vacuum which will be fired
if number of inserted tuples becomes big. Now I don't think that the  idea is
useful for two reason: for small number of tuples completion is a cheap and it
should be called before ginbulkdelete. IMHO, it's enough to add an optional
method to pg_am (aminsertcleanup, per Tom's suggestion). This method will be
called before ambulkdelete and amvacuumcleanup. Opinions, objections, suggestions?

On presentation some people were interested on how our changes affect the
search speed after rows insert. The tests are below: We use the same tables as
in presentation and measure search times ( after insertion of some rows ) before
and after vacuum. All times are in ms. Test tables contain 100000 rows, in the
first table the number of elements in array is 100 with cardinality = 500,
second - 100 and 500000, last - 1000 and 500.

Insert 10000 into table with 100000 rows (10%)
                  |    v && '{v1}'   |
-----------------+---------+--------+ found
                  | novac-d |  vac-d |  rows
-----------------+---------+--------+-------
n:100,  c:500    |   118   |    35  | 19909
n:100,  c:500000 |    95   |   0.7  |    25
n:1000, c:500    |   380   |   79   | 95211


Insert 1000 into table with 100000 rows (1%)
                  |    v && '{v1}'   |
-----------------+---------+--------+ found
                  | novac-d |  vac-d |  rows
-----------------+---------+--------+-------
n:100,  c:500    |    40   |    31  | 18327
n:100,  c:500000 |    13   |   0.5  |    26
n:1000, c:500    |   102   |    71  | 87499

Insert 100 into table with 100000 rows (0.1%)
                  |    v && '{v1}'   |
-----------------+---------+--------+ found
                  | novac-d |  vac-d |  rows
-----------------+---------+--------+-------
n:100,  c:500    |    32   |    31  | 18171
n:100,  c:500000 |   1.7   |   0.5  |    20
n:1000, c:500    |    74   |    71  | 87499

Looking at result it's easy to conclude that:
  - time of search pending list is O(number of inserted rows), i.e., search time
    is equal to (time of search in GIN) + K1 * (number of inserted rows after the
    last vacuum).
  - search time is O(average length of indexed columns). Observations made above
    is also applicable here.
  - significant performance gap starts around 5-10% of inserts or near 500-1000
    inserts.  This is very depends on specific dataset.

Notice, that insert performance to GIN was increased up to 10 times. See
exact results in presentation.

Do we need to add option to control this (fast insertion) feature?
If so, what is a default value? It's not clear to me.

Note: These patches are mutually exclusive because they touch the same pieces
of code and I'm too lazy to manage several depending patches. I don't see any
problem to join patches to one, but IMHO it will be difficult to review.

--
Teodor Sigaev                                   E-mail: teodor@sigaev.ru
                                                    WWW: http://www.sigaev.ru/