Re: [INTERFACES] Re: [HACKERS] changes in 6.4

> The results are profound. Queries that used to scan tables because of the
> ORs, now make use of any indexes. Thus, the size of the table has virtually
> no effect on performance. Furthermore, queries that used to crash the
> backend, now run in under a second.
>
> Currently the down sides are:
> 1. If there is no usable index, performance is significantly worse. The
> patch does not check to make sure that there is a usable index. I could use
> some pointers on this.
>
> 2. Small tables are actually a bit slower than without the patch.
>
> 3. Not very elegant. I am looking for a more generalized solution.
> I have lots of ideas, but I would need to know the backend much better before
> attempting any of them. My favorite idea is before cnfify(), to factor the
> OR terms and pull out the constants into a virtual (temporary) table spaces.
> Then rewrite the query as a join. The optimizer will (should) treat the new
> query accordingly. This assumes that an efficient factoring algorithm exists
> and that temporary tables can exist in the heap.

OK, I have an idea. Just today, we allow:

select *
from tab1
where val in (
select x from tab2
union
select y from tab3
)

How about if instead of doing:

select * from tab1 where val = 3
union
select * from tab1 where val = 4
...

you change it to:

select * from tab1 where val in (
select 3
union
select 4
)

This may be a big win. You aren't running the same query over and over
again, with the same joins, and just a different constant.

Let me know.

If it fails for some reason, it is possible my subselect union code has
a bug, so let me know.

--
Bruce Momjian | 830 Blythe Avenue
maillist(at)candle(dot)pha(dot)pa(dot)us | Drexel Hill, Pennsylvania 19026
+ If your life is a hard drive, | (610) 353-9879(w)
+ Christ can be your backup. | (610) 853-3000(h)