Re: GiST splitting on empty pages

On 10/03/2014 05:03 AM, Andrew Gierth wrote:
> This is from Bug #11555, which is still in moderation as I type this
> (analysis was done via IRC).
>
> The GiST insertion code appears to have no length checks at all on the
> inserted entry. index_form_tuple checks for length <= 8191, with the
> default blocksize, but obviously a tuple less than 8191 bytes may
> still not fit on the page due to page header info etc.
>
> However, the gist insertion code assumes that if the tuple doesn't fit,
> then it must have to split the page, with no check whether the page is
> already empty.
>
> [script to reproduce]

Thanks for the analysis!

> Suggested fixes (probably all of these are appropriate):
>
> 1. gistSplit should have check_stack_depth()
>
> 2. gistSplit should probably refuse to split anything if called with
> only one item (which is the value being inserted).
>
> 3. somewhere before reaching gistSplit it might make sense to check
> explicitly (e.g. in gistFormTuple) whether the tuple will actually
> fit on a page.
>
> 4. pg_trgm probably should do something more sensible with large leaf
> items, but this is a peripheral issue since ultimately the gist core
> must enforce these limits rather than rely on the opclass.

Fixed, I did 1. and 2. Number 3. would make a lot of sense, but I
couldn't totally convince myself that we can safely put the check in
gistFormTuple() without causing some cases to fail that currently work.
I think the code sometimes forms tuples that are never added to the
insert as such, used only to "union" them to existing tuples on internal
pages. Or maybe not, but in any case, the check in gistSplit() is enough
to stop the crash.

- Heikki