Optimize raster vectorization

[knutole]

Currently the vectorize raster function (ST_DumpAsPolygon) is taking around 20-30 seconds to process a 1.6MB raster.

It's all being done one 1 CPU. Possibly there's a way to cut this job into pieces and do them parallel, utilizing 6-8 CPU's.

[strk]

We should be already cutting the raster into pieces of 128x128 pixels at import time.
The scandinavia snow raster cuts in 96 rows here, at full resolution.
It takes ~28 seconds on my system to dump each of those 96 rasters into 517,825 polygons and write them into a new table using a (ST_DumpAsPolygons(rast)).* query. The time halves removing the ().* syntax, as it is known to run the internal function once for each output field (2 fields in our case). Is that enough of an improvement ? Where's the code doing the vectorialization ?

[strk]

I found two places, both using the ().* syntax:
src/pile.js:420
src/vectorize_raster.sh