[postgis-devel] Re: geometry stats

[Mark Cave-Ayland]

Hi strk,

> > Looking at the comments in postgis_estimate.c, do you really get 
> > different query results when uncommenting your 'selectivity 
> = 0.9'? Is 
> > this true if you try different values, say from 0.1 to 0.9?
> 
> Yes, box_overlap() is not the same as geometry_overlap().
> There are no traces of box_overlap() in postgis code though,
> so the late broke might depends on external factors.

Yeah, box_overlap() is a routine from the internal PostgreSQL geometric
functions (see src/backend/utils/adt/geo_ops.c). The current way it
works is that a BOX3D is converted into a BOX and then box_overlap() is
called. Yuck. The fix in the thread I mentioned before was that I wrote
my own equivalent of the contains() operator and added it into PostGIS
instead of calling the PostgreSQL contains/contained functions which
just appeared to be plain broken.

So can you clarify again: are you seeing different results depending
upon whether a sequential scan/index scan is being used, or do you get
different results with the same query plan but returning a different
value for the selectivity?

> > I suppose the other possibility is that some memory is being 
> > overwritten where it shouldn't be. I've had another quick 
> look at your 
> > new code and the only thing I've noticed at first sight is that 
> > PostgreSQL passes float4 values by reference(!) and not by 
> value which 
> > may cause the
> > get_attstatlot()/free_attrstatslot() not to handle memory as they
> > should. Then again I would expect this to have been more 
> likely to seg
> > fault rather than upset the query....

Actually don't worry about this - it looks as if this is only the case
for stavalues and not stanumbers - so we should be safe.

> I actually get a random segfault, isn't this a 
> get_attstatslot bug, anyway ? 

This is in the estimation function, right? Can you narrow it down to a
particular area at all, i.e. if you just have the function do nothing
except call get_attstatslot(), free_attstatslot() and return the default
selectivity, does it still crash? I think this would be a good test.

> I guess I'll be waiting for you on this :)

I'll try and help out as much as I can in the meantime.


Cheers,

Mark.

---

Mark Cave-Ayland
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