Re: [HACKERS] Use of Indicies ...

Tom Lane wrote:
> 
> The Hermit Hacker <scrappy@hub.org> writes:
> > There is an index on all three conditions in the WHERE clause:
> > Yet EXPLAIN shows:
> > Aggregate  (cost=2.05 rows=1 width=4)
> -> Index Scan using referrer_link_counter_id on referrer_link  (cost=2.05 rows=1 width=4)
> 
> > Why does EXPLAIN only show the use of one of the indices, why counter_id
> > and why not all three?
> 
> Indexscans only know how to use one index at a time.

would it make sense to implement a new (?) "join" strategy for evaluating
AND's 
that gets just tids from affected indices and then performs a multiset AND on
them
sorts the result and then uses the it to access the actual rows.

> 
> Do you have reason to think that one of the other indexes would have
> been cheaper?
> 

It could be faster if one could actually do the above.

It's just another of these unintuitive things I mentioned in the "Happy
dropping" 
thread - common sense guess of optimiser behaviour is not what it actually
done. 

--------------
Hannu