btw, I think it's worth to add link to src/backend/access/gist/README
file, which we updated recently.
Oleg
On Thu, 20 Oct 2005, Tom Lane wrote:
> [ getting back to this documentation issue finally ]
>
> Teodor Sigaev <teodor@sigaev.ru> writes:
>> I disagree with last affirmation: inner pages of index contains fair union of
>> keys and enough helpful to select. Mailware ( http://www.pgsql.ru/db/mw )
>> sucsessfully use combined GiST index (date, tsvector) for searching.
>
>> GiST's split algorithm is good for unique leading keys, not so bad for small
>> number of non-unique values and bad for all equals leading key. But "bad" means
>> that itsn't optimal as picksplit for other keys may be. If there is several keys
>> which can be moved on left or right page without changing union of first key for
>> each page then GiST try put its on page (left or right) with smallest penalty
>> calculated by other keys. This algorithm is very similar to defining page to put
>> tuple with normal processing (without page split).
>
>> With unique leading key GiST's split is fully similar to BTree - it looks only
>> at leading key, but gistchoose isn't. Gistchoose (gistutil.c:622) chooses child
>> with smallest penalty and it looks to other keys if several leading keys has the
>> same penalty. In a GiST tree different keys may have the same penalty value with
>> new key.
>
> OK, how about this text then?
>
> A multicolumn GiST index can only be used when there is a query condition
> on its leading column. Conditions on additional columns restrict the
> entries returned by the index, but the condition on the first column is the
> most important one for determining how much of the index needs to be
> scanned. A GiST index will be relatively ineffective if its first column
> has only a few distinct values, even if there are many distinct values in
> additional columns.
>
> regards, tom lane
>
> ---------------------------(end of broadcast)---------------------------
> TIP 9: In versions below 8.0, the planner will ignore your desire to
> choose an index scan if your joining column's datatypes do not
> match
>
Regards,
Oleg
_____________________________________________________________
Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
Sternberg Astronomical Institute, Moscow University (Russia)
Internet: oleg@sai.msu.su, http://www.sai.msu.su/~megera/
phone: +007(095)939-16-83, +007(095)939-23-83