Re: GIN improvements part 1: additional information

On 01/17/2014 08:49 PM, Alexander Korotkov wrote:
> On Fri, Jan 17, 2014 at 10:38 PM, Heikki Linnakangas <
> hlinnakangas(at)vmware(dot)com> wrote:
>
>> On 01/17/2014 01:05 PM, Alexander Korotkov wrote:
>>
>>> Seems to be fixed in the attached version of patch.
>>> Another improvement in this version: only left-most segments and further
>>> are re-encoded. Left part of page are left untouched.
>>
>> I'm looking into this now. A few quick notes:
>>
>> * Even when you don't re-encode the whole page, you still WAL-log the
>> whole page. While correct, it'd be a pretty obvious optimization to only
>> WAL-log the modified part.
>
> Oh, I overlooked it. I wrote code in ginRedoInsertData which finds
> appropriate place fro data but didn't notice that I still wrote whole page
> to xlog record.

Yeah. I think ginRedoInsertData was too cute to be bug-free. I added an
explicit unmodifiedsize field to the WAL record, so that
ginRedoInsertData doesn't need to calculate it.

>> * When new items are appended to the end of the page so that only the last
>> existing compressed segment is re-encoded, and the page has to be split,
>> the items aren't divided 50/50 on the pages. The loop that moves segments
>> destined for the left page to the right won't move any existing, untouched,
>> segments.
>
> I think this loop will move one segment. However, it's too small.

Implemented this.

I noticed that the gin vacuum redo routine is dead code, except for the
data-leaf page handling, because we never remove entries or internal
nodes (page deletion is a separate wal record type). And the data-leaf
case is functionally equivalent to heap newpage records. I removed the
dead code and made it more clear that it resembles heap newpage.

Attached is a yet another version, with more bugs fixed and more
comments added and updated. I would appreciate some heavy-testing of
this patch now. If you could re-run the tests you've been using, that
could be great. I've tested the WAL replay by replicating GIN operations
over streaming replication. That doesn't guarantee it's correct, but
it's a good smoke test.

- Heikki