[postgis-tickets] [PostGIS] #3555: Parallelization of PostGIS raster ST_Union aggregate

[PostGIS]

#3555: Parallelization of PostGIS raster ST_Union aggregate
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 Reporter:  robe         |      Owner:  dustymugs
     Type:  enhancement  |     Status:  new
 Priority:  medium       |  Milestone:  PostGIS Future
Component:  raster       |    Version:  trunk
 Keywords:               |
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 I experimented with this and sadly I think this is a no go for PostgreSQL
 9.6, though I'm keeping ourt hope for PostgreSQL 9.7.

 The reason I think it's a no go (unless we revise postgis raster ST_Union
 to use similar approach to ST_MemUnion (which might result in more loss
 than gain) is because I don't think the PostgresQL parallel aggregate
 infrastructure can support aggregates that use state transitions.

 Reason I thin that is that function like string_agg, array_agg don't
 support parallelization in 9.6
 http://www.hagander.net/talks/PostgreSQL_9.6.pdf#38

 I'm guessing its because they use that internal state transition stuff
 that we employ with our raster aggregates.

 I'll try to still experiment with a couple of approaches before I give up
 on 9.6 parallelization for raster.

 The ST_MapAlgebra functions look like good candidates for parallelization,
 so I'll add that aas a separate ticket, as I think that one would be
 easier to swing.

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Ticket URL: <https://trac.osgeo.org/postgis/ticket/3555>
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