Yes, I know the 2 syntax provide a different result: one provides the 6 meaningful lines, the left join do add 5 lines with a count of 0...
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On Sat, Jan 7, 2023 at 8:46 PM Marc Millas <marc.millas@xxxxxxxxxx> wrote:
Hi,postgres 12, postgis 3.0I have a small table A, 11 rows with a varchar column x and a geometry column y.gist index on the geometry column.the geometry do contains multipolygons (regions on a map)I have a second table B , same structure, around 420 000 rows.no index,the geometry do contains points.all geometries are on 4326 srid.If i ask to count points in each multipolygons:select A.x, count(B.x) from A, B where st_within(B.y, A.y) group by A.x;it takes 11 seconds (everything in shared buffers).If I do the very same thing as:select A.x, count(B.x) from A left join B on st_within(B.y, A.y) group by A.x;same result, but 85 seconds (every thing in shared buffers, again)if I redo asking with explain analyze, buffers, the plan is very different.if I do create a gist index on geometry column of the big table, both syntax takes 21 seconds.I get the feeling I am missing something.. (at least 2 things...)can someone shed some light ??thanks
