Why must Table-Valued Parameters be READONLY

This question asked by interviewer. Why must Table-Valued Parameters be READONLY. Let’s see why it is read only. Table-Valued Parameters are mandatory READONLY because SQL Server passes them as immutable, streaming row sets optimized for concurrency, plan stability, and minimal logging. Allowing writes would break pipelining, require locking and logging, and invalidate execution plan assumptions. If modification is required, the TVP must be copied into a temp table.

When you pass a TVP as below

DECLARE @MyTVP MyTableType;

Then SQL Server Materializes it in memory (or tempdb if large) and treats it like a structured input stream and shares it safely across execution contexts. It is NOT a normal table variable.

TVPs are passed as a serialized row set stream. SQL Server can pipeline rows into execution plan
to avoid locking and avoid logging. If writes were allowed then it breaks streaming and would require full materialization locking.

Concurrency safety: Multiple threads may read TVP simultaneously. If we allowing writes than it would cause the Race conditions and Latches/locks.

Let’s see the demo

Creating table type

CREATE TYPE dbo.OrderType AS TABLE (     OrderID INT,     Amount DECIMAL(10,2) );

Now creating SP to consume this TT.

CREATE PROCEDURE dbo.usp_TestTVP     @Orders dbo.OrderType READONLY AS BEGIN     SELECT * FROM @Orders; END;

SP created successfully.

Let’s try to remove READONLY  and then creating the SP.

CREATE PROCEDURE dbo.usp_TestTVP_RemRO     @Orders dbo.OrderType AS BEGIN     SELECT * FROM @Orders; END;

Getting an error.

SQL Server enforces it.

Let’s try to insert/update the TT inside the SP.

CREATE PROCEDURE dbo.usp_TestTVP_insert     @Orders dbo.OrderType READONLY AS BEGIN INSERT INTO @Orders VALUES (3, 100); SELECT * FROM @Orders; END;

Getting an error.

When we need to update the data of Table type first, we need to create temp table or variable table and store TT data into it and the update temp table or variable table.