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.


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