[raster] Add a note on ST_Union(raster) slowness

[pracine]

So we receive a bit less complains

And give some promises that WWDBIANR (we will do better in a next release) or WWDBITNR…

[robe]

done at r9508. I also put in a note to use ST_Clip first and then union.

I discovered when creating my report images that if I clip the rasters to my polygon area of interest and then union the raster shards, it's way faster than trying to union the intersecting rasters and then clipping to my geometry of interest.

For example for my o_2 the difference was 3.5 seconds vs. 8 seconds if you (clip + union) vs. (union + clip).

For the hi-res, the timing was more starkling — something like 9 seconds vs. 30 seconds.

[robe]

r9508

[robe]

hmm r9502 I meant.