[postgis-tickets] [PostGIS] #4310: Improve MVT vectortile_layer_combine to remove duplicates

[PostGIS]

#4310: Improve MVT vectortile_layer_combine to remove duplicates
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 Reporter:  Algunenano   |      Owner:  Algunenano
     Type:  enhancement  |     Status:  assigned
 Priority:  medium       |  Milestone:  PostGIS 3.0.0
Component:  postgis      |    Version:  trunk
 Keywords:               |
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 In the current parallel implementation of ST_AsMVT we combine layers by
 copying both keys and values from both layers on top of each other.

 This is fast but introduces the possibility of duplicates (both in keys
 and values), which is discouraged in the v2 spec. This means that the
 final tiles end up being bigger than they should, and might create issues
 with decoders that incorrectly rely on the keys to be unique (e.g:
 https://github.com/mapbox/vector-tile/issues/55).

 We should look for a performant way to merge the layers removing
 duplicates, if possible.

 Also, that part of the code isn't currently being tested, so we'd need to
 look for the proper way to do it (raising costs for the test or disabling
 certain plans).

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Ticket URL: <https://trac.osgeo.org/postgis/ticket/4310>
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