Question 405360 in PostgreSQL Question posted in
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PostgreSQL Check Right before Transaction closes

i have the following tables:

tenants:

  • tenantId (UUID)
  • invoiceAddress (TEXT)

packages:

  • packageId
  • needsInvoice

tenants_and_packages:

  • tenantsAndPackagesId
  • tenantId
  • packageId

This is a M2N-Mapping between Tenants and Packages.

What is the problem?

I want to achieve the following "Check"-Logic. I want to make sure that when I insert/update a tenant or package, it will be checked whether the tenant needs to have the column invoiceAddress set or not (it depends from the needsInvoice column of the Packages-Table.

Now, because sometimes I only edit the tenant, sometimes only the package and sometime both right after each other. But every time it runs in the context of a migration. Therefore, the "check" must be executed right before the transaction would finish (commit). Because, when I want to edit a tenant not to have the invoiceAddress set, because in the next step (same transaction-context) I want to set the package’s needsInvoice to false, this kind of "check" cannot be executed right after the tenant has changed, since it would immediatly abort the transaction.

Thanks in advance! If any help for understanding is required, lmk!

  • Phynae

  • Hence the fact, that constraints do not allow subqueries or any dynamic logic, it won’t work with them as far as I know.

  • Using triggers and functions is also not working since they are executed right before or after the row was inserted/updated.

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2

Answers


  1. 0

    You could use a deferred constraint trigger:

    CREATE CONSTRAINT TRIGGER ...
AFTER INSERT OR UPDATE ON ...
DEFERRABLE INITIALLY DEFERRED
FOR EACH ROW EXECUTE ...

    Such a trigger will run right before the commit of the transaction, just like you want.

    But you should be aware that such a trigger will always suffer from race conditions if there are concurrent data modifications, because data modifications by a concurrent statement are not visible in the trigger function.

    As detailed in my article, you either need the SERIALIZABLE isolation level or locking to serialize the operations if you want such triggers to work reliably.

    Perhaps you can redesign your data model so that there are no constraints between columns of different tables…

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  2. 0

    You can achieve the desired behavior using a PL/pgSQL block that encapsulates the logic within a transaction. By using BEGIN, COMMIT, and ROLLBACK, you can control the transaction flow based on whether the needsInvoice condition is met. This approach allows you to perform the necessary checks without the need for triggers.
    Example PL/pgSQL Block
    Here’s how you can implement this logic in a PL/pgSQL block:

    DO $$
DECLARE
   tenant_id UUID; -- Replace with actual tenant ID
   package_id INT; -- Replace with actual package ID
   invoice_address TEXT; -- Replace with actual invoice address
   needs_invoice BOOLEAN;
BEGIN
   -- Example values (replace with actual values)
   tenant_id := 'your-tenant-uuid'; -- Replace with actual UUID
   package_id := 1; -- Replace with actual package ID
   invoice_address := '123 Main St'; -- Replace with actual invoice address -- Start the transaction
   BEGIN
       -- Check the needsInvoice value for the specified package
       SELECT needsInvoice INTO needs_invoice
       FROM packages
       WHERE packageId = package_id;

       -- If needsInvoice is true, check the invoiceAddress
       IF needs_invoice IS TRUE THEN
           IF invoice_address IS NULL OR invoice_address = '' THEN
               RAISE EXCEPTION 'Invoice address must be set for tenant % when package % needs invoice', tenant_id, package_id;
           END IF;
       END IF;

       -- Perform the insert/update operations
       -- Example: Insert or update tenant
       INSERT INTO tenants (tenantId, invoiceAddress) 
       VALUES (tenant_id, invoice_address)
       ON CONFLICT (tenantId) DO UPDATE SET invoiceAddress = invoice_address;

       -- Example: Update package if needed
       UPDATE packages
       SET needsInvoice = TRUE -- or FALSE based on your logic
       WHERE packageId = package_id;

       -- Commit the transaction if all checks pass
       COMMIT;
   EXCEPTION
       WHEN OTHERS THEN
           -- Rollback the transaction in case of any exception
           ROLLBACK;
           RAISE; -- Re-raise the exception for further handling/logging
   END; 
END $$;

    Explanation
    Transaction Block:
    The BEGIN block starts a transaction. If any part of the transaction fails, it will trigger the EXCEPTION block.

    Check Logic:
    The code checks the needsInvoice value for the specified package.
    If needsInvoice is true, it checks if the invoice_address is set. If not, it raises an exception.

    Insert/Update Operations:
    You can perform your INSERT or UPDATE operations for the tenants and packages tables as needed.

    Commit/Rollback:
    If all checks pass and the operations are successful, the transaction is committed.

    If any exception occurs (including the raised exception for missing invoice address), the transaction is rolled back to maintain data integrity.
    Benefits of This Approach

    Control: You have full control over the transaction flow without relying on triggers.

    Flexibility: You can easily adapt the logic to handle various scenarios within the same transaction context.
    Error Handling: You can customize the error handling and logging as needed.

    This approach effectively ensures that the invoiceAddress is set according to the needsInvoice requirement and rolls back the entire transaction if the conditions are not met.

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