Re: impact join syntax ?? and gist index ??

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...
...

Marc MILLAS
Senior Architect
+33607850334
www.mokadb.com

On Sat, Jan 7, 2023 at 8:46 PM Marc Millas <marc(dot)millas(at)mokadb(dot)com> wrote:

> Hi,
>
> postgres 12, postgis 3.0
>
> I 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
>
>
> Marc MILLAS
> Senior Architect
> +33607850334
> www.mokadb.com
>
>