Re: GIN improvements part 1: additional information

Last version of patch is attached.
WAL is debugged here. How it's actually working.
Also there was some refactoring, commenting and README update.
Benchmark scripts are also updated. Updated version is attached.
I did some benchmark comparison with different count of index entries. See
results is tables below.

event | master | 16-entries | 32-entries
| 64-entries | 128-entries |
-----------------------+-----------------+-----------------+-----------------+-----------------+-----------------+
index_build | 00:01:50.042658 | 00:01:53.79182 |
00:01:55.647561 | 00:01:52.677095 | 00:01:58.723898 |
index_build_recovery | 00:00:19 | 00:00:06 | 00:00:05
| 00:00:06 | 00:00:06 |
index_update | 00:05:18.215707 | 00:06:09.404842 |
00:05:49.015441 | 00:05:39.987697 | 00:05:38.723376 |
index_update_recovery | 00:01:48 | 00:01:51 | 00:01:48
| 00:01:47 | 00:01:47 |
search_new | 00:25:21.481699 | 00:23:23.59775 |
00:25:13.943362 | 00:23:58.633514 | 00:22:30.763075 |
search_updated | 00:25:57.622592 | 00:25:29.867388 |
00:27:33.683614 | 00:25:17.565714 | 00:26:29.333003 |

label | size | 16-entries | 32-entries | 64-entries |
128-entries |
---------------+------------+------------+------------+------------+-------------+
new | 884514816 | 417013760 | 421240832 | 430350336 |
450994176 |
after_updates | 1595252736 | 711368704 | 719380480 | 735682560 |
774275072 |

It's probably an option to select 64 entries instead of 32.
There is still some regression in update speed. However, there is also room
for improvement patch. It searches item index entries 2 times on insert: in
dataLocateLeafItem and dataPlaceToPage. We can save full results of
dataLocateLeafItem, but it require some rework of gin btree interface:
store not only offset of item.

------
With best regards,
Alexander Korotkov.