Hi,
it seems the usage of ST_Envelope(geom) might cause troubles after all,
and we probably don't want that enabled by default (ufff....)
I was playing with a China wide OSM download, and querying a small
area on Hong Kong, after running "vacuum analyze", and ensuring
the indexes are there, gives me the following:
osm=# \timing
Timing is on.
osm=# SELECT
"id","name",encode(ST_AsBinary(ST_Force_2D("geometry")),'base64') as
"geometry" FROM "import"."osm_roads" WHERE "geometry" &&
ST_GeomFromText('POLYGON ((12686441.96088916 2574141.200893554,
12686441.96088916 2575819.375869846, 12688531.453260839 2575819.375869846,
12688531.453260839 2574141.200893554, 12686441.96088916
2574141.200893554))', 3857);
Time: 1,649 ms
osm=# SELECT
"id","name",encode(ST_AsBinary(ST_Force_2D("geometry")),'base64') as
"geometry" FROM "import"."osm_roads" WHERE ST_envelope("geometry") &&
ST_GeomFromText('POLYGON ((12686441.96088916 2574141.200893554,
12686441.96088916 2575819.375869846, 12688531.453260839 2575819.375869846,
12688531.453260839 2574141.200893554, 12686441.96088916
2574141.200893554))', 3857);
Time: 1076,481 ms
That is, wrapping the geometry with ST_Envelope makes the query 500 times
slower... erk!
I looked into it, and it seems that for this database wrapping with
ST_Envelope makes the DB
use sequential search:
----------------------------------------------------------------------------------------------
Seq Scan on osm_roads (cost=0.00..122097.24 rows=229722 width=243)
Filter: (st_envelope(geometry) &&
'0103000020110F00000100000005000000A09ABF3E8D32684146E1B6999EA34341A09ABF3E8D326841CC801CB0E5A64341E01C816E92336841CC801CB0E5A64341E01C816E9233684146E1B6999EA34341A09ABF3E8D32684146E1B6999EA34341'::geometry)
(2 rows)
whilst without ST_Envelope around, it's a index search:
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on osm_roads (cost=4.61..180.05 rows=42 width=243)
Recheck Cond: (geometry &&
'0103000020110F00000100000005000000A09ABF3E8D32684146E1B6999EA34341A09ABF3E8D326841CC801CB0E5A64341E01C816E92336841CC801CB0E5A64341E01C816E9233684146E1B6999EA34341A09ABF3E8D32684146E1B6999EA34341'::geometry)
-> Bitmap Index Scan on osm_roads_geom (cost=0.00..4.60 rows=42
width=0)
Index Cond: (geometry &&
'0103000020110F00000100000005000000A09ABF3E8D32684146E1B6999EA34341A09ABF3E8D326841CC801CB0E5A64341E01C816E92336841CC801CB0E5A64341E01C816E9233684146E1B6999EA34341A09ABF3E8D32684146E1B6999EA34341'::geometry)
(4 rows)
So... I guess we should flip the sys var the other way? Or make it into
a store parameter again (sigh)?
I believe road networks are going to be more popular/common than
datasets with huge polygons, that why I'm suggesting to turn it off
by default.
Cheers
Andrea
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