Inserting polygon centroids into an additional column in PostGIS

Question

I'm trying to calculate the x and y centroids for a table using ST_Centroid. The documentation says that it returns the geometric center of a geometry. I first made a column X:

ALTER TABLE "test_table" ADD X double precision;

and was then planning on storing the x component of the centroid there. My code to calculate the centroid is as follows:

select
  ST_Centroid(a.the_geom) 
FROM 
  "test_table" AS a

However this query returns the following result which I don't understand:

"01010000006BAEC2FBDFC61E415B3E9E2946520541"
"0101000000E6ABADEBFEC51E4102E9A95858520541"
"01010000003233333321C61E410BD7A37061520541"
"0101000000F710787300C61E4177C8C783D94E0541"

My second problem with this approach is trying to store the result. When I tried to insert this result into Column X I could not figure out the correct syntax. What I tried using was:

INSERT INTO "test_table" (X)
VALUES ( ST_Centroid(a.the_geom) ),
FROM "test_table" AS a

and also

INSERT INTO "test_table" (X)
VALUES ( SELECT ST_Centroid(a.the_geom) 
FROM "test_table" AS a)

but neither of these work.

My two very basic questions are (1) what is the value being returned by ST_Centroid, and (2) how I can I insert the result into an existing table?

Answer 1

If you want x and y columns:

ALTER TABLE "test_table" ADD x double precision;
ALTER TABLE "test_table" ADD y double precision;
UPDATE "test_table" SET x = ST_X(ST_Centroid(the_geom));
UPDATE "test_table" SET y = ST_Y(ST_Centroid(the_geom));

If you rather want a geometry column (adjust schema name and CRS to fit your needs):

SELECT AddGeometryColumn ('test_schema','test_table','centroid_geom',4326,'POINT',2);
UPDATE "test_table" SET centroid_geom = ST_Centroid(the_geom);

Answer 2

You must use ST_X function.

Assuming that you want to fill a column of a table that already has a geometry column with the x coordinate of the centroid you can use something like:

UPDATE test_table
SET x = ST_X(ST_Centroid(the_geom));

If you need something different post the complete structure of the table if you need more accurate help.

Answer 3

It is a Geometry (a Point in fact). What you are seeing is the text representation of the point as Well Known Binary (WKB) for debugging purposes you'd probably want well known text (WKT) (see https://postgis.net/docs/using_postgis_dbmanagement.html#OpenGISWKBWKT),

select
  ST_AsText(ST_Centroid(a.the_geom) )
FROM 
  "test_table" AS a

Will show what you want to see.

To do the addition you need a geometry column rather than a double. For this you need to use addGeometryColumn which handles all the bookkeeping needed for you (things like adding the column to the geometry columns metadata etc).