Different SRID shown in QGIS and PostGIS for the same data

Question

I have data stored in PostGIS. When I view its properties in QGIS, it says:

SRS: EPSG:4326 - WGS 84

However, when I want retrieve the SRID value directly using the Find_SRID function, it returns 0. And by looking at the data, SRID is really 0 in PostGIS.

Query:

select * from geometry_columns;

Returns:

f_table_name | f_geometry_column | coord_dimension | srid |  type

-----------------+------------------+--------------+-------------------

vector       | wkb_geometry      |               2 |    0 | POINT

vector       | wkb_geometry      |               2 |    0 | POINT

buff_out     | wkb_geometry      |               2 |    0 | POLYGON

buff_out     | wkb_geometry      |               2 |    0 | POLYGON

Question: Why is SRID for the same data different in QGIS and in PostGIS? How do I get the correct SRID value from PostGIS?

I am using PostGIS 9.5.

Answer 1

QGIS serves mostly as a visualization tool, where you are importing information from a Geospatial Database, this means it can't update the DB as when you change the layer property, it only affects that layer that lives in that QGIS project.

To update the SRID in PostGIS you should declare it as:

SELECT UpdateGeometrySRID('vector','wkb_geometry',4326);

Answer 2

To update the SRID in PostgreSQL/PostGIS you can run:

ALTER TABLE vector ALTER COLUMN wkb_geometry TYPE geometry(POINT,4326) using ST_SetSRID(wkb_geometry, 4326);

This does the exact same thing as YPCA's answer, but is standard SQL.