POSTGIS - Count points that are inside a polygon and aggregate them by a common feature

Question

I'm experiencing an issue in counting and aggregate a set of points by one of their features (in this case the "group") that are contained in a polygon.

DATA

I have a table extracted from a csv that is like this into a table :

csv ( 
      id integer, 
      group character varying,
      lat float,
      long float
    ) 

  id |    group     |   lat   |   long    |
-----+--------------+---------------------+
  1  |      A       |   ...   |    ...    |
  2  |      A       |   ...   |    ...    |
  3  |      C       |   ...   |    ...    |
  4  |      C       |   ...   |    ...    |      
  5  |      D       |   ...   |    ...    |    
  6  |      B       |   ...   |    ...    |

and I have also a table with a set of adjacent polygons that should contain some of the points.

boundary (
          gid integer,
          geom geometry(POLYGON,4326)
         )

 gid |         geom          |   
-----+-----------------------+
  1  |      polygon(...)     |
  2  |      polygon(...)     |
  3  |      polygon(...)     |

DESIRED OUTPUT

My intention is to create a "stats" table with contained the details of how many points are "inside" / "outside" the merged polygons and aggregate these points by "group"

stats (
       s_id integer,
       group character varying,
       inside integer,
       outside integer
      )

  s_id |    group     |   inside   |   outside    |
-------+--------------+---------------------------+
   1   |      A       |     ...    |      ...     |
   2   |      B       |     ...    |      ...     |
   3   |      C       |     ...    |      ...     |
   4   |      D       |     ...    |      ...     |      

WHAT I DID (wrong)

just for the "inside" column

UPDATE stats
SET inside = agg.total
FROM (
      --- get the *group* name and count all the *points*
      SELECT points.group, count ( points.geom ) total 
      FROM ( 
             ---  Creating the Points from the *CSV* table
             SELECT group, ST_SetSRID(St_Point(long,lat), 4326) geom
             FROM csv
           ) AS points,
           ( 
             --- merge all the bounduaries together 
             SELECT ST_SetSRID(St_Union(geom), 4326) geom
             FROM bounduaries
           ) AS bounduary
      --- check wich point is inside the bounduary
      WHERE St_Within(points.geom,bounduary.geom)
      --- group each selected point by it group
      GROUP BY points.group
      ) agg 
--- update according the group
WHERE stats.group = agg.group

when I'm running this full query the server continues to run like a sort of infinite loop, but if I'm running a small part of it I'm getting results in few seconds. There are a couple of millions of points, but all the previews task that I did were performed pretty fast.

THE QUESTION

Where I'm doing it wrong? Someone, could explain it to me?

THE EXPLAIN

Update on stats  (cost=107640.76..107661.08 rows=11 width=124)
  ->  Hash Join  (cost=107640.76..107661.08 rows=11 width=124)
        Hash Cond: (stats.group = (agg.group)::bpchar)
        ->  Seq Scan on stats  (cost=0.00..17.40 rows=740 width=82)
        ->  Hash  (cost=107640.73..107640.73 rows=3 width=44)
              ->  Subquery Scan on agg  (cost=107640.67..107640.73 rows=3 width=44)
                    ->  HashAggregate  (cost=107640.67..107640.70 rows=3 width=18)
                          ->  Nested Loop  (cost=10.45..105221.63 rows=241903 width=18)
                                ->  Seq Scan on csv  (cost=0.00..60024.78 rows=1562678 width=0)
                                ->  Nested Loop  (cost=10.45..29570.08 rows=1 width=18)
"                                      Join Filter: ((st_setsrid(st_point(lng,lat), 4326) && (st_setsrid(st_union((geom)::geometry), 4326))) AND _st_contains((st_setsrid(st_union((geom)::geometry), 4326)), st_setsrid(st_point(lng,lat), 4326)))"
                                      ->  Seq Scan on csv  (cost=0.00..4249.80 rows=92880 width=18)
                                      ->  Aggregate  (cost=10.45..10.46 rows=1 width=32)
                                            ->  Seq Scan on bounduary  (cost=0.00..10.30 rows=30 width=32)

Answer 1

Using the comments that I've received on the post I figure it out how to solve it. @John suggested me to use CTE (Common Table Expression) and trick the query analyzer that was freezing before. I've used as a reference this two posts, how to use it and what is a CTE. After that, it works fine!

WITH points AS ( SELECT group, ST_SetSRID(St_Point(lng, lat), 4326) AS geom
                 FROM csv
                ),
     bounduary AS ( SELECT ST_SetSRID(St_Union(geom), 4326) AS geom
                    FROM boundaries
                )
UPDATE stats
SET inside = agg.total
FROM  (
       SELECT  points.group as group, coalesce(count( points.pt ),0) as num
       FROM points,border
       WHERE St_Within(points.geom,border.geom)
       GROUP BY points.group
      ) agg
WHERE agg.group = stat.group;

EXPLAIN

GroupAggregate  (cost=486420.42..486426.72 rows=200 width=40)
  Group Key: points.group
  CTE points
    ->  Seq Scan on csv  (cost=0.00..70237.24 rows=1721749 width=34)
  CTE border
    ->  Aggregate  (cost=4.14..4.15 rows=1 width=32)
          ->  Seq Scan on bonduaries  (cost=0.00..4.11 rows=11 width=32)
  ->  Sort  (cost=486420.42..486421.85 rows=574 width=64)
        Sort Key: points.group
        ->  Nested Loop  (cost=0.00..486394.11 rows=574 width=64)
"              Join Filter: ((border.geom ~ points.geom) AND _st_contains(border.geom, points.geom))"
              ->  CTE Scan on bonduary  (cost=0.00..0.02 rows=1 width=32)
              ->  CTE Scan on points  (cost=0.00..34434.98 rows=1721749 width=64)