We are going to talk about SQL Views and Generated Columns in postgresql.

Introduction

Views

Have you heard about views in SQL? Maybe! Since it has been here since approximatly forever.

For those who don’t what a view is, here is a example :

CREATE TABLE person (
    p_id uuid primary key,
    p_firstname text,
    p_lastname text,
    p_whatever_you_need_to_know text,
);

CREATE VIEW v_person AS SELECT p_firstname, p_lastname FROM person;

So views are basically a SELECT.

On postgresql1 you also have MATERIALIZED VIEW which are compute when you create the view, not when you request

CREATE TABLE person (
    p_id uuid primary key,
    p_firstname text,
    p_lastname text,
    p_salary int,
);

-- here insert an insane number of person into the db so any request on it will take > 1sec

CREATE VIEW v_person AS SELECT count(p_id), sum(p_salary), avg(p_salary),  stddev(p_salary) FROM person;
-- it should be long
SELECT * FROM v_person;

CREATE MATERIALIZED VIEW vm_person AS SELECT count(p_id), sum(p_salary), avg(p_salary),  stddev(p_salary) FROM person
-- it should be instant
SELECT * FROM vm_person;

Both are great, you just choose where computation takes place. That’s the beauty of it.

Generated Columns

Generated Columns have been introduced in postgresql 12.

They let you have a column computed at insert/update time. Not at query time.

CREATE TABLE person (
    p_id uuid primary key,
    p_firstname text,
    p_lastname text,
    p_hashcode text generated always as (sha256(decode(p_firstname || p_lastname, 'escape'))) stored
    );

I’m a geo guy, so let’s do some geo-usefull stuff with generated column that we usually do via views and/or trigger :

CREATE TABLE trip (
    t_id uuid primary key,
    t_date timestamptz,
    t_geom geometry(LineString, 2154),
    t_distance float generated always as (st_length(t_geom)) stored
);

INSERT INTO trip (t_id, t_date, t_geom) VALUES ('bc2c2ec4-a4c3-4df8-922c-b17277e8eb03', now(), 'LINESTRING(0 0, 1 1, 2 1, 2 2)')
SELECT * FROM trip; 
                 t_id                 |            t_date             |          st_astext          |    t_distance     
--------------------------------------+-------------------------------+-----------------------------+-------------------
 bc2c2ec4-a4c3-4df8-922c-b17277e8eb03 | 2020-05-01 10:00:35.580758+00 | LINESTRING(0 0,1 1,2 1,2 2) | 3.414213562373095

You cannot do everything

Be careful. function used in generated column need to be IMMUTABLE !

For example this code cannot work, because now() function is not immutable.

CREATE TABLE person (
    p_id uuid primary key,
    p_firstname text,
    p_lastname text,
    p_created_or_updated_at timestamptz generated always as (now()) stored
    );

Conclusion

With postgresql release 12, you have more control than ever to choose when you pay the price for data computation. At insert/update or at query time.


  1. On other DB too ! ↩︎