[BSD] I/O sebesseg merese

[Laszlo Nagy]

Antal Rutz írta:
> Laszlo Nagy wrote:
>   
>> Elnézést kérek mindenkitől akit ezzel zavartam. Átmentem egy másik 
>> listára ezzel. :-)
>>     
>
> Azert a dolog most mar engem is kezd erdekelni.
> A vegeredmenyt post-old ide vissza majd a vegen, ha lehet.
> Kosz!
>   
A pgsql-performance listan indult egy thread errol. Vegul Tom Lane 
valaszolt (egy main postgresql developer). Bemasolom alabb:

> Alvaro Herrera <alvherre at commandprompt.com> writes:
>   
>> > Are you saying that an indexscan "Filter" only acts after getting the
>> > heap tuple?
>>     
>
> Correct.
>
>   
>> > If that's the case, then there's room for optimization
>> > here, namely if the affected column is part of the index key, then we
>> > could do the filtering before fetching the heap tuple.
>>     
>
> Only if the index is capable of disgorging the original value of the
> indexed column, a fact not in evidence in general (counterexample:
> polygons indexed by their bounding boxes in an r-tree).  But yeah,
> it's interesting to think about applying filters at the index fetch
> step for index types that can hand back full values.  This has been
> discussed before --- I think we had gotten as far as speculating about
> doing joins with just index values.  See eg here:
> http://archives.postgresql.org/pgsql-hackers/2004-05/msg00944.php
> A lot of the low-level concerns have already been dealt with in order to
> support bitmap indexscans, but applying non-indexable conditions before
> fetching from the heap is still not done.
>
> 			regards, tom lane
Ez tok jo lenne. Join-olni ugy, hogy csak az indexeket hasznalja fol, es 
a tablat nem. :-) Valoszinuleg sokkal hatekonyabb. Sajnos ez jelenleg 
nem mukodik, es ugy nez ki hogy egy ideig nem is fog. :-(

Udv,

   Laci