Are you navigating the complexities of migrating from SQL Server to PostgreSQL? This essential blog series kicks off by demystifying a crucial difference between the two database engines: how SELECT and RETURN statements behave within Stored Procedures. Dive in to understand this key distinction and make your migration journey smoother.
Even though there are some products that can help you do the migration (Google DMS, AWS DMS, Ispirer and more) and the fact that AI tools can help you as well (see for example in both Google and Ispirer links), good understanding of the key engine differences is important.
Similarities between PostgreSQL and SQL Server
Let’s start with the similar part. Functions in both databases can return a single value or a single record set. If you try to return more than one value — in both engines you will get an error “Select statements included within a function cannot return data to a client”.
Now, let’s move to the Stored Procedure part. If I write the same functions as stored procedures, there will be a big difference.
In this case, for SQL Server the syntax is valid. The SELECT 6 clause returns a result set (one column) to the client, while RETURN 7 returns a separate, integer status code that is captured by the @r variable. A single SQL Server stored procedure can return both.
Now for the surprise (at least for SQL Server experts)
SQL Server’s stored procedures offer a flexible way to return a result set, a scalar return code, or even multiple result sets from a single call. This is where a key behavioral difference with PostgreSQL emerges.
In PostgreSQL, a PROCEDURE is a separate concept. It’s designed for transactional logic and cannot return a value or a result set to the client.
- The statement SELECT 6 is invalid in a PostgreSQL procedure because it attempts to return a result set.
- The RETURN 7 statement is also invalid because a procedure cannot return a value.
If your SQL Server procedure returns one result set or a return value only, you can replace the SQL Server stored procedure by a PostgreSQL function. However, if there are multiple result sets (of one value!) or even one result set and a return value (like in the example below), you might find the OUT option in PostgreSQL as a nice solution.
To summarize the solution:
The solution moves from two separate result sets (a table and a scalar) to one combined result set (a table with two columns). The solution is based on:
- Simplicity: It keeps all the logic in one function.
- Clarity: The function’s signature clearly defines what the caller will receive.
- Best Practice: This is a more idiomatic way to handle multiple return values in PostgreSQL than trying to mimic SQL Server’s specific behavior.
I hope you understand the different behaviour of the engines. It is not that one approach is better. However, migrating SQL Server procedures is not always easy (migrating functions is much simpler).
Is this the full picture? No. We’ve covered a relatively simple case, but what if a stored procedure returns multiple, distinct tables with different schemas? Stay tuned for the next blog where we’ll explore that more complex migration scenario.
Source Credit: https://medium.com/google-cloud/select-and-return-in-stored-procedures-sql-server-vs-postgres-part-1-f3be9a27fe13?source=rss—-e52cf94d98af—4
