Merge buffers PostGIS

"I did this to be able to create polygon around the points which are within 10 m." - I'll give it a shot at what I think you want to do, that is "clustering all your points with a cluster distance of 10m and create a convex hull around each cluster"...

To simply draw the minimal enclosing geometry around each cluster, you can do the following, using ST_ClusterWithin and ST_ConvexHull:

WITH
  clst AS (
    SELECT ST_ClusterWithin(<geometry_column>, 10) arr
    FROM <your_point_table>
  )

SELECT a.cluster_id,
       ST_ConvexHull(ST_CollectionHomogenize(a.geom)) AS geom
FROM clst,
     unnest(clst.arr) WITH ORDINALITY a(geom, cluster_id)

Note: this will return the minimal enclosing geometry of each cluster - that can also be a POINT geometry for a single point that is his own cluster, or a LINESTRING geometry for two points being a cluster! It's a little tricky (some say unadvisable) to work with mixed geometry types in the same table.

As an option, you could only select those clusters that are actually enclosed by a POLYGON:

WITH
  clst AS (
    SELECT ST_ClusterWithin(<geometry_column>, 10) arr
    FROM <your_point_table>
  ),

  clst_encl AS (
    SELECT a.cluster_id,
           ST_ConvexHull(ST_CollectionHomogenize(a.geom)) AS geom
    FROM clst,
         unnest(clst.arr) WITH ORDINALITY a(geom, cluster_id)
  )

SELECT *
FROM clst_encl
WHERE GeometryType(geom) = 'POLYGON'

with the minor backdraw that the extracted array's ordinals I used as cluster_id are not strictly consecutive due to rows being omitted.

You could also opt for simply assigning the cluster_id to your original points:

WITH
  clst AS (
    SELECT ST_ClusterWithin(<geometry_column>, 10) arr
    FROM <your_point_table>
  ),

  clst_ext AS (
    SELECT a.cluster_id,
           ST_CollectionHomogenize(a.geom) AS geom
    FROM clst,
         unnest(clst.arr) WITH ORDINALITY a(geom, cluster_id)
  )

SELECT ce.cluster_id,
       pt.*
FROM <your_point_table> AS pt
JOIN clst_ext AS ce
  ON pt.<geometry_column> && ce.geom

with the minor backdraw that this obviously returns no polygons...

Does that sound like what you need?


EDIT:

As @dbaston thankfully pointed out and I keep forgetting, ST_ClusterDBSCAN is way better suited here;

SELECT ST_ClusterDBSCAN(pt.<geometry_column>, 10, 1) OVER() AS cluster_id,
       *
FROM <your_point_layer> AS pt

to assign the cluster_ids to your points;

SELECT cluster_id,
       ST_ConvexHull(ST_Collect(geom)) AS geom
FROM (
  SELECT ST_ClusterDBSCAN(<geometry_column>, 10, 1) OVER() AS cluster_id,
         <geometry_column> AS geom
  FROM <your_point_layer>
) AS cluster
GROUP BY cluster_id
ORDER BY cluster_id

to get the minimal enclosing geometry of each cluster.