If you create a series of circles/buffers in QGIS, what type of geometry do they go into PostGIS as?
SELECT ST_Buffer(ST_MakePoint(0, 0), 1) will return a POLYGON with 32 sides, or a triacontadigon. If you wan't more or less sides, you can specify
num_seg_quarter_circle in ST_Buffer (the default is 8). A series of unioned POLYGONS may result in a MULTIPOLYGON.
If you really need a circle or curved geometry, you can investigate ST_LineToCurve, however these types of geometry often have poor or limited support, so I recommend avoiding this.
Depends on the input.