Identity Inserts in Databases: The Debate Between Control and Flexibility (And What Benjamin Franklin Has to Do With It)

Have you ever been wondering why MSSQL doesn't allow inserting IDs into tables with autoincrement primary keys?

If you've ever worked with databases, chances are you’ve come across tables that have an IDENTITY or SERIAL column. These special columns automatically generate unique values, making them perfect for primary keys. But there’s a quirk in how different databases handle these auto-generated IDs, particularly when trying to insert your own values into these columns. Microsoft SQL Server (MSSQL) and PostgreSQL approach this problem differently, and understanding these differences is essential for avoiding pitfalls.

In this article, we will explore why MSSQL prevents you from inserting into tables with auto-increment primary keys, how PostgreSQL allows it (but with a caveat), and the potential issues you might face if you're not careful.

MSSQL's Strictness with Identity Columns

MSSQL enforces strict rules when it comes to inserting into columns marked with IDENTITY. By default, MSSQL's IDENTITY column generates a sequential integer automatically for each new row. Attempting to manually insert a value into this column will throw an error unless you explicitly enable this option.

Let's consider a simple example. First, we'll create a table with an IDENTITY column:

CREATE TABLE Employees (
    EmployeeID INT IDENTITY(1,1) PRIMARY KEY,
    Name NVARCHAR(50)
);

In this table, the EmployeeID column is an IDENTITY column, automatically incrementing for every new row.

Now, if we try to insert a row and specify an ID ourselves:

INSERT INTO Employees (EmployeeID, Name) VALUES (1, 'John Doe');

MSSQL will throw an error:

Cannot insert explicit value for identity column in table 'Employees' when IDENTITY_INSERT is set to OFF.

Why MSSQL Prevents Direct Inserts

By design, MSSQL protects the integrity of the IDENTITY column to prevent unintended conflicts or duplicates. If you were to insert values manually into an identity column, it could disrupt the sequence and cause primary key violations. MSSQL maintains strict control over its IDENTITY column to ensure that the values remain unique and sequential, which is crucial for indexing and performance optimization.

However, if you really need to insert a custom value, MSSQL provides a workaround:

SET IDENTITY_INSERT Employees ON;

INSERT INTO Employees (EmployeeID, Name) VALUES (1, 'John Doe');

SET IDENTITY_INSERT Employees OFF;

This allows you to insert custom values into the IDENTITY column, but notice how you need to explicitly turn this option on and off. It’s a deliberate, somewhat cumbersome process to ensure that you don’t accidentally disrupt the sequence.

PostgreSQL's Flexibility with Serial Columns

PostgreSQL, on the other hand, is more lenient with its SERIAL columns (the PostgreSQL equivalent of MSSQL’s IDENTITY). In PostgreSQL, you can insert values into a SERIAL column without any special permissions or configurations.

Let's create a similar table in PostgreSQL:

CREATE TABLE employees (
    employee_id SERIAL PRIMARY KEY,
    name VARCHAR(50)
);

Now, let's attempt to insert a row while providing our own employee_id:

INSERT INTO employees (employee_id, name) VALUES (1, 'Jane Doe');

Unlike MSSQL, PostgreSQL will allow this operation without error. However, this is where a subtle yet critical difference comes into play: PostgreSQL does not automatically update the sequence associated with the SERIAL column when you insert a custom value.

The Risk of Sequence Mismatch

PostgreSQL manages the values in a SERIAL column using a sequence. When you insert a custom value, the sequence is not adjusted to match this new value. This can lead to future insertion errors once the sequence reaches a conflicting value.

For example, after the previous insert, if you try to add another row without specifying an ID:

INSERT INTO employees (name) VALUES ('Alice Smith');

You might encounter an error like:

ERROR: duplicate key value violates unique constraint "employees_pkey"
DETAIL: Key (employee_id)=(1) already exists.

This happens because the sequence is still set to generate IDs starting from the initial value (1 in this case), not recognizing that 1 was already manually inserted.

Why PostgreSQL Doesn’t Automatically Update the Sequence

The reason PostgreSQL doesn’t automatically update the sequence value on manual inserts is based on a principle of flexibility. PostgreSQL assumes that if you're manually inserting values, you have a specific purpose for doing so and want to retain control over the sequence. Automatically adjusting the sequence could have unintended side effects, particularly in scenarios where custom logic depends on specific ID values.

However, if you need to manually sync the sequence with the maximum value in the table, you can do so using the setval function:

SELECT setval('employees_employee_id_seq', (SELECT MAX(employee_id) FROM employees));

This sets the sequence to the current maximum ID, ensuring the next insert uses a unique, incremented value.

Comparison: Safety vs. Flexibility

Aspect	MSSQL	PostgreSQL
Insert into Auto-ID	Not allowed unless IDENTITY_INSERT ON	Allowed without restrictions
Sequence Adjustment	Automatic with every insert	Not automatic on manual inserts
Error Handling	Immediate error on manual insert	Possible conflicts on subsequent inserts
Rationale	Data integrity, prevent conflicts	Flexibility, developer control

Conclusion

MSSQL’s approach is focused on maintaining data integrity and avoiding conflicts at all costs. It requires explicit steps to insert values into IDENTITY columns, ensuring that the sequence is never unintentionally disrupted.

PostgreSQL, on the other hand, offers more flexibility but leaves room for potential pitfalls. While it allows manual inserts into SERIAL columns, it expects developers to handle sequence adjustments if needed. This flexibility can be beneficial in some scenarios but demands careful handling to prevent conflicts down the line.

Benjamin Franklin once said:

Those who would give up essential liberty to purchase a little temporary safety, deserve neither liberty nor safety

So do you think if that saying might be applicable here and PostgreSQL's flexible approach should prevail? Or is Microsoft's caution justified in this case?