Re: GSoC 2017: weekly progress reports (week 8)

On Thu, Sep 28, 2017 at 2:22 PM, Alexander Korotkov <
a(dot)korotkov(at)postgrespro(dot)ru> wrote:

> On Fri, Jul 28, 2017 at 7:58 AM, Shubham Barai <shubhambaraiss(at)gmail(dot)com>
> wrote:
>
>> I am attaching a patch for predicate locking in SP-Gist index
>>
>
> I took a look over this patch.
>
> newLeafBuffer = SpGistGetBuffer(index,
>> GBUF_LEAF | (isNulls ? GBUF_NULLS : 0),
>> Min(totalLeafSizes,
>> SPGIST_PAGE_CAPACITY),
>> &xlrec.initDest);
>> PredicateLockPageSplit(index,
>> BufferGetBlockNumber(current->buffer),
>> BufferGetBlockNumber(newLeafBuffer));
>>
>
> You move predicate lock during split only when new leaf page is
> allocated. However SP-GiST may move items to the free space of another
> busy page during split (see other branches in doPickSplit()). Your patch
> doesn't seem to handle this case correctly.
>

I've more thoughts about this patch.

+ * SPGist searches acquire predicate lock only on the leaf pages.
> + During a page split, a predicate lock is copied from the original
> + page to the new page.

This approach isn't going to work. When new tuple is inserted into
SP-GiST, choose method can return spgAddNode. In this case new branch of
the tree is added. And this new branch could probably be used by scans we
made in the past. Therefore, you need to do predicate locking for internal
pages too in order to detect all the possible conflicts.

------
Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company