impact join syntax ?? and gist index ??

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