Re: BUG #12292: index row size 1480 exceeds maximum 1352 for index

On 12/24/2014 06:44 PM, Tom Lane wrote:
> robert(dot)thaler(at)cellent(dot)at writes:
>> I tried to upgrade from postgres 9.3.5 and imported a database export
>> created by pg_dump. the import shows the following error:
>> ERROR: index row size 1480 exceeds
>> maximum 1352 for index "idx_sm_post_content"
>
> I've looked into this (thanks to Robert for the test data), and the short
> answer is that commit 36a35c55 approximately halved GinMaxItemSize:
>
> #define GinMaxItemSize \
> - MAXALIGN_DOWN(((BLCKSZ - SizeOfPageHeaderData - \
> - MAXALIGN(sizeof(GinPageOpaqueData))) / 3 - sizeof(ItemIdData)))
> + Min(INDEX_SIZE_MASK, \
> + MAXALIGN_DOWN(((BLCKSZ - SizeOfPageHeaderData - \
> + MAXALIGN(sizeof(GinPageOpaqueData))) / 6 - sizeof(ItemIdData))))
>
> What was the rationale for deciding that GIN has to be able to fit six
> tuples per page??? This is going to create serious dump/reload hazards
> for a lot of users.

Oh. I went to look at the discussions on this patch, and that change was
present already in the very early versions that were posted around. I
don't know the reason for that, and I failed to catch it before
committing; it certainly looks bogus.

I'll revert that, and add a comment above GinMaxItemSize explaining that
we need to fit at least 3 items on each page. If such a comment had been
in place, I'm sure someone would've caught this earlier.

Actually, I think we would only need to fit 2 items on each page in GIN.
In b-tree, we need to fit three: the high key, and two data keys. But in
the GIN entry tree, we don't store high keys explicitly, we just use the
rightmost key on the page. That works because we never delete items from
the entry tree. (I don't dare to change the above to /2, nevertheless)

- Heikki