postgresql - Handing null primary key in postgres - Stack Overflow

In our project, we are trying to combine multiple tables into a single table. The table has ukey and effective_date are PKs with not null constraint.

The structure works for below data as effective dates are different:

Ukey effective date combination:

ukey	effective_date	settle_date	price
u123	3/3/2025	3/8/2025	1.21215
u123	3/4/2025	3/9/2025	1.23

In our project, we are trying to combine multiple tables into a single table. The table has ukey and effective_date are PKs with not null constraint.

The structure works for below data as effective dates are different:

Ukey effective date combination:

ukey	effective_date	settle_date	price
u123	3/3/2025	3/8/2025	1.21215
u123	3/4/2025	3/9/2025	1.23

The requirement is for same ukey and effective_dates, there can be multiple settle_dates. So if I wanted to make it work for same ukey and effective_date, I will have to add settle_date as PK and make it non null. However there can be certain records where settle_date is null so adding settle_date with the data shown below won't work:

Ukey effective date settle_date combination:

ukey	effective_date	settle_date	price
u123	3/3/2025	3/8/2025	1.21215
u123	3/3/2025	3/9/2025	1.23
u456	3/3/2025

Two solutions I have thought are:

1. Always populate dummy value in settle_date eg 12/31/2999 making settle_date as PK and non null. And wherever its applicable during data load, my table will have actual settle_date.
2. Use sequence as GENERATED BY DEFAULT AS IDENTITY and use it as PK rather than settle_date. However there would be lot of INSERTS if I don't handle updates properly in the code.

Any other solution to this issue?

• postgresql

Share Improve this question Follow edited Mar 5 at 3:44 Guillaume Outters 2,71711 gold badge1717 silver badges2121 bronze badges asked Mar 3 at 6:56 SachinSachin 6566 bronze badges 1

• Can there be multiple rows with the same ukey and effective_date, if they all have no settle_date? If so, are there any other features that distinguish them? – Bergi Commented Mar 3 at 14:26

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2 Answers 2

Sorted by: Reset to default 2

A UNIQUE constraint is almost as potent as a PRIMARY KEY, with the crucial difference that it allows null values, which are considered to be distinct values as per the SQL standard - by default. Since Postgres 15 this leeway can be cut by adding the NULLS NOT DISTINCT clause. "Certain records where settle_date is null" does not clarify to me whether multiple null values for the same (ukey, effective_date) should be allowed or not.

ALTER TABLE tbl ADD CONSTRAINT tbl_u_e_s_uniq UNIQUE (ukey, effective_date, settle_date);

Or:

ALTER TABLE tbl ADD CONSTRAINT tbl_u_e_s_uniq UNIQUE NULLS NOT DISTINCT (ukey, effective_date, settle_date);

fiddle

See:

• Create unique constraint with null columns
• How does PostgreSQL enforce the UNIQUE constraint / what type of index does it use?

Depending on undisclosed data types and predominant queries, the order of columns in the constraint may be optimized. (But the set of columns is required to enforce your requirement.)

I would probably add a surrogate PK (serial or IDENTITY column) additionally. See:

• Auto increment table column

A third option would be to degrade your constraint to a "simple" UNIQUE CONSTRAINT.

Just like with a primary key, you'll get:

• consistency
• an index
• ability to foreign key to it (" foreign key must reference columns that either are a primary key or form a unique constraint")

create unique index on t(ukey, effective_date, coalesce(settle_date, '1900-01-01')); -- Personal opinion: choosing a past date is less risky than a future one; but to evacuate all risks, add a ", settle_date is null" to the index.

(see fiddle for demo)

(your question is in fact universal: see nearly same question on Reddit or for SQL Server)