RE: Index Skip Scan

> > > On Sun, Apr 05, 2020 at 04:30:51PM +0530, Dilip Kumar wrote:
> > >
> > > I was just wondering how the distinct will work with the "skip scan"
> > > if we have some filter? I mean every time we select the unique row
> > > based on the prefix key and that might get rejected by an external
> > > filter right?
> >

Yeah, you're correct. This patch only handles the index conditions and doesn't handle any filters correctly. There's a
checkin the planner for the IndexScan for example that only columns that exist in the index are used. However, this
checkis not sufficient as your example shows. There's a number of ways we can force a 'filter' rather than an 'index
condition'and still choose a skip scan (WHERE b!=0 is another one I think). This leads to incorrect query results.
 

This patch would need some logic in the planner to never choose the skip scan in these cases. Better long-term solution
isto adapt the rest of the executor to work correctly in the cases of external filters (this ties in with the previous
visibilitydiscussion as well, as that's basically also an external filter, although a special case).
 
In the patch I posted a week ago these cases are all handled correctly, as it introduces this extra logic in the
Executor.

-Floris