Re: Making strxfrm() blobs in indexes work

On Thu, Jan 30, 2014 at 4:34 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Quite aside from the index bloat risk, this effect means a 3-4x reduction
> in the maximum string length that can be indexed before getting the
> dreaded "Values larger than 1/3 of a buffer page cannot be indexed" error.
> Worse, a value insertion might well succeed, with the failure happening
> only (much?) later when that entry is chosen as a page split boundary.

That's not hard to prevent. If that should happen, we don't go with
the strxfrm() datum. We have a spare IndexTuple bit we could use to
mark when the optimization was applied. So we consider the
appropriateness of a regular strcoll() or a strxfrm() in all contexts
(in a generic and extensible manner, but that's essentially what we
do). I'm not too discouraged by this restriction, because in practice
it won't come up very often.

>> I'm sure anyone that has read this far knows where I'm going with
>> this: why can't we just have strxfrm() blobs in the inner pages,
>> implying large savings for a big majority of text comparisons that
>> service index scans, without bloating the indexes too badly, and
>> without breaking anything? We only use inner pages to find leaf pages.
>> They're redundant copies of the data within the index.
>
> It's a cute idea though, and perhaps worth pursuing as long as you've
> got the pitfalls in mind.

I'll think about pursuing it. I might prefer to declare it as fair
game for anyone else that wants to do it.


-- 
Peter Geoghegan