On Mon, Jan 17, 2011 at 07:35:52AM +0100, Magnus Hagander wrote:
> On Mon, Jan 17, 2011 at 06:51, Itagaki Takahiro
> <itagaki.takahiro@gmail.com> wrote:
> > On Mon, Jan 17, 2011 at 04:05, Andy Colson <andy@squeakycode.net> wrote:
> >> This is a review of:
> >> https://commitfest.postgresql.org/action/patch_view?id=468
> >>
> >> Purpose:
> >> ========
> >> Equal and not-equal _may_ be quickly determined if their lengths are
> >> different. ? This _may_ be a huge speed up if we don't have to detoast.
> >
> > We can skip detoast to compare lengths of two text/bytea values
> > with the patch, but we still need detoast to compare the contents
> > of the values.
> >
> > If we always generate same toasted byte sequences from the same raw
> > values, we don't need to detoast at all to compare the contents.
> > Is it possible or not?
>
> For bytea, it seems it would be possible.
>
> For text, I think locales may make that impossible. Aren't there
> locale rules where two different characters can "behave the same" when
> comparing them? I know in Swedish at least w and v behave the same
> when sorting (but not when comparing) in some variants of the locale.
>
> In fact, aren't there cases where the *length test* also fails? I
> don't know this for sure, but unless we know for certain that two
> different length strings can never be the same *independent of
> locale*, this whole patch has a big problem...
Just to be clear, the code already has these length tests today. This patch
just moves them before the detoast.
