Re: [HACKERS] OR with multi-key indexes

Bruce Momjian wrote:
>
> > > > II. Extend multi-key indexing: (y = 1 or y = 2) could be
> > > > qualified by index access methods itself because of Y is
> > > > one of index keys. Only first key would be used for finding
> > > > index tuples but additional qualification could decrease
> > > > number of heap_fetch calls and this would be nice!
> > >
> > > > This feature would be also usefull for:
> > > >
> > > > create index on table (a,b,c);
> > > > select * from table where a = 1 and c = 2;
> > > >                                     ^^^^^
> > > >     additional qualification would be performed on index level
> > > >
> > > > Personally, I would like to see II implemented first because
> > > > of it works for both query examples.
> > >
> > > Doesn't the existing code already use both keys in the above query.
> > > What is gained by moving this to the index access methods?
> >
> > I hadn't time to implement this year ago...
> >
> > Let's say we have 1000 tuples with a = 1 and only 10 with
> > a = 1 and c = 2 - currently, all 1000 index tuples will be returned
> > to Executor and all corresponding 1000 heap tuples will be fetched...
> > Having this feature, only 10 index tuples would be returned
> > and heap_fetch would be called 10 times only.
>
> OK, stupid question, but why do we have multi-key indexes then?  Just to
> allow UNIQUE index failure?

In the example above only 1st and _3rd_ index keys are used
in WHERE and so only 1st key will be used by index.
For cases like WHERE a = 1 and b = 3 index will use
1st and 2nd keys.
For cases like WHERE a = 1 and b = 3 and c = 2 index will use
all three keys.

Extending II means not using 3rd key to _find_ index tuples
but using 3rd key to reduce # of index tuples returned
to executor. Btree will read all tuples with a = 3 but
will not return index tuple with a = 3 and c = 0 (ie).

Vadim