> Hannu Krosing wrote:
> > > The days where every release fixed server crashes, or added a feature
> > > that users were 'screaming for' may be a thing of the past.
> > Is anyone working on fixing the exploding optimisations for many OR-s,
> > at least the canonic case used by access?
> > My impression is that this has fallen somewhere between
> > insightdist and Vadim.
> This is really big for the ODBCers. (And I suspect for JDBCers too.) Many
> desktop libraries and end-user tools depend on this "record set" strategy to
> operate effectively.
> I have put together a workable hack that runs just before cnfify(). The
> option is activated through the SET command. Once activated, it identifies
> queries with this particular multi-OR pattern generated by these RECORD SET
> strategies. Qualified query trees are rewritten as multiple UNIONs. (One
> for each OR grouping).
> 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.
> SELECT ... FROM tab WHERE
> (var1 = const1 AND var2 = const2) OR
> (var1 = const3 AND var2 = const4) OR
> (var1 = const5 AND var2 = const6)
> SELECT ... FROM tab, tmp WHERE
> (var1 = var_x AND var2 = var_y)
> var_x | var_y
David, where are we on this? I know we have OR's using indexes. Do we
still need to look this as a fix, or are we OK. I have not gotten far
enough in the optimizer to know how to fix the cnf'ify problem.
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)
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