[postgis-devel] Regression performance.

[Sandro Santilli]

On Thu, Feb 09, 2012 at 12:20:45PM -0800, David Zwarg wrote:
> Hello,
> 
> I was profiling the performance of the changes that happened in r9112, and
> it turned out to be very significant to pass and evaluate integers in the
> prepared expression. Sticking with float8 values for the x and y positions
> of the pixel contain significant performance benefits (although
> counter-intuitive).
> 
> I've changed the x and y positions in 1 raster map algebra back to float
> values for performance reasons.
> 
> These are my metrics for running 5 sets of 10 rasters, each 500x500 pixels,
> from one of the SRTM tiles linked to by the WKTRaster tutorial page. Times
> are in milliseconds. The first column is revision 9112 (removal of strstr
> in the pixel loop, using integer as x and y pixel values), the second
> column is that same revision (with a modification to use float8 instead of
> int32 for x and y pixel coordinates), the third column is revision 9111
> (using strstr in the pixel loop), the fourth column is the incorporation of
> this change into the HEAD revision as of this morning (more memory
> allocations have been moved out of the pixel loop).

The numbers for those like me not willing to fire a graphical browser:

  r9112            r9112-int       r9111           r9137
  ----------------------------------------------------------
   9839.746        8473.403        9094.589        7820.646
   6228.067        5329.813        5763.88         4738.372
   8749.516        7441.936        7893.893        6866.745
  13618.762       11254.392       12177.314       10780.401
   6233.851        5442.746        5730.231        4767.192

I'm really surprised. Which version of PostgreSQL were you using ?
I'm still not convinced we should pretend numbers are float though.
It was changed back for a reason:
http://trac.osgeo.org/postgis/ticket/1557

--strk; 

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