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From: Heikki Linnakangas <hlinnakangas@vmware.com>
To: Alexander Korotkov <aekorotkov@gmail.com>
Cc: Tomas Vondra <tv@fuzzy.cz>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: GIN improvements part 1: additional information
Date: Fri, 17 Jan 2014 20:38:22 +0200
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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


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