Is it possible to create a PostGIS sequence to populate Area column of a polygon

Question

I've created Postgres/PostGIS sequences before to assign unique gid values when a feature is created in QGIS but is it possible to create a sequence that will automatically populate an area column? Equally would it be possible for a sequence to be created to calculate the length of a line and even better the X Y values of a point?

Answer 1

Here's a sample I use to update both lon/lat columns and x-y columns each time a point is added

CREATE OR REPLACE FUNCTION update_lonlat_coords()
  RETURNS trigger AS
$BODY$
DECLARE g integer;
BEGIN
    g:=NEW.gauge_id;
    UPDATE rain_gauges 
        SET x_coord=ST_X(the_geom), y_coord=ST_Y(the_geom)    
        WHERE gauge_id=g;
    UPDATE rain_gauges 
        SET lon=ST_X(ST_TRANSFORM(the_geom,4326)), lat=ST_Y(ST_TRANSFORM(the_geom,4326))    
        WHERE gauge_id=g;
    RETURN NULL;
END;
$BODY$
  LANGUAGE plpgsql VOLATILE
  COST 100;

Answer 2

triggers can do that for you. Search for example, you have a plsql function that uses x y data to fill geometry column. Régis

Answer 3

A sequence isn't what you want here. You are looking for Triggers and Rules

Example coming