I've read a lot from both camps in the surrogate vs. natural key debate. What I am struggling to understand, and what seems to be glossed over, especially in explaining how surrogate keys are more efficient, is how do referencing tables resolve a surrogate key?

Say, I am using an ORM in an application, and have a userID in an application object. now we want to insert into a SelectedTask table and a SubTask table that are defined as follows (ignore the awkwardness of the real word example):

CREATE TABLE SelectedTask(
userID INT References User(userID)
taskNumber INT,
name TEXT ,
jobSiteID INT References Site(jobSiteID),

assignee INT References User(assignee),
subTaskID INT References Subtasks(subTaskID),
timeAllowed INT,
reportTo INT References Group(reportTo),
-- how do we reference SelectedTask?

If we use a surrogate key for the SelectedTask table, how do we use that in the SubTask table? If we use natural keys, the primary key could simply be a compound primary key of userId and taskNumber, and we could insert into both at the same time assuming we have programmatically gathered all of the pieces.


1 Answer 1


A proper Postgres schema for your example could look like this:

CREATE TABLE selected_task(
  selected_task_id serial PRIMARY KEY  -- surrogate PK
, user_id          int REFERENCES users(user_id)
, task_number      int
, name             text
, jobsite_id       int REFERENCES site(jobsite_id)

CREATE TABLE sub_task(
  sub_task_id      serial PRIMARY KEY
, selected_task_id int REFERENCES selected_task  -- this is how
, assignee         int REFERENCES users(assignee)
, time_allowed     int
, report_to        int REFERENCES groups(report_to)

INSERT in both tables at the same time?

Use a data-modifying CTE with the RETURNING clause, or two INSERT statements in a transaction with lastval() (which is the Postgres equivalent for SCOPE_IDENTITY() in SQL server @Aaron mentioned in his comment):

Related answer with code example for either method.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.