Re: GIN improvements part 1: additional information

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.

* 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.

> !     /*
> !      * Didn't fit uncompressed. We'll have to encode them. Check if both
> !      * new items and uncompressed items can be placed starting from last
> !      * segment of page. Then re-encode only last segment of page.
> !      */
> !     minNewItem = newItems[0];
> !     if (nolduncompressed == 0 &&
> !             ginCompareItemPointers(&olduncompressed[0], &minNewItem) < 0)
> !         minNewItem = olduncompressed[0];

That looks wrong. If I'm understanding it right, it's trying to do 
minNewItem = Min(newItems[0], olduncompressed[0]). The test should be 
"nolduncompressed > 0 && ..."

No need to send a new patch, I'll just fix those while reviewing...

- Heikki