[PostGIS] #6145: ST_AsMVTGeom: skip wagyu for polygons that stay simple in the tile

[PostGIS]

#6145: ST_AsMVTGeom: skip wagyu for polygons that stay simple in the tile
-----------------------------+---------------------------
 Reporter:  johncarmack1984  |      Owner:  pramsey
     Type:  enhancement      |     Status:  new
 Priority:  medium           |  Milestone:  PostGIS 3.8.0
Component:  postgis          |    Version:  3.6.x
 Keywords:  mvt              |
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 ST_AsMVTGeom sends every polygon through wagyu
 (`lwgeom_wagyu_clip_by_box`, from `mvt_clip_and_validate`), even one that
 sits inside the tile and is still valid after the snap to the tile grid.
 On Berlin's buildings at z14 tile 8802/5371 (2,128 polygons), wagyu was
 68% of the database's CPU in a perf profile of PostGIS 3.6.4.

 For a single ring inside the clip box that's simple after the snap, and
 whose edges stay at least half a pixel from every vertex they don't end
 at, wagyu returns the same ring: positive area in tile coordinates,
 collinear vertices removed, starting where its sweep closes the ring. I
 have a patch that finds those rings with exact integer tests and writes
 them the way wagyu would. Everything else still goes to wagyu, and the
 output is identical byte for byte.

 Tiles per second, Berlin buildings, Martin 2.0's tile query, pgbench with
 8 clients on 6 CPUs, master 83e5285:

 ||= Tiles =||= master =||= patch =||= =||
 || z14 8802/5371 || 492 || 1,200 || 2.45x ||
 || random z14 tiles || 1,310 || 3,194 || 2.44x ||
 || random z16 tiles || 11,794 || 19,367 || 1.65x ||
 || random z18 tiles || 37,650 || 42,515 || 1.12x ||

 Compared against wagyu, 40,276,961 rings that took the fast path came out
 identical: Berlin data at several zooms, extents and buffers, 2M random
 rings, and an exhaustive enumeration of small rings on small grids. All
 94,380 Berlin tiles I built at z8-z18 have the same md5 with and without
 it.

 This is for 3.8. Pull request to follow.
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Ticket URL: <https://trac.osgeo.org/postgis/ticket/6145>
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