Postgresql – Ideal way to return a scalar from function (RETURN vs. SELECT)

This may be more work but it should get (approximately) what you are after:

1 - Create a new table that has just your PK from the main table and the updated function.

2 - Drop your old column

3 - Rename your old table to something like TableName_Base

4 - Create a VIEW that joins your main table to your lookup table, with the fields in the order that you want to see them. Name the view whatever your original table name was.

This will potentially create some issues with INSERTS and UPDATES though since you'll be accessing via a view. To deal with that you can either insert into the table directly, bypassing the view.

Scalar functions are called once-per-row, when called as part of a query.

Consider the following example.

1. Create a new, blank database for our tests:

   USE master;
   IF EXISTS (SELECT 1 FROM sys.databases d WHERE d.name = 'mv')
   BEGIN
       ALTER DATABASE mv SET SINGLE_USER WITH ROLLBACK IMMEDIATE;
       DROP DATABASE mv;
   END
   GO
   CREATE DATABASE mv;
   GO
   

2. Create a table, a multi-statement function, and a table-valued-function:

   USE mv;
   GO
   CREATE TABLE dbo.t
   (
       t_id int NOT NULL
           CONSTRAINT PK_t
           PRIMARY KEY CLUSTERED
   );
   GO
   
   CREATE FUNCTION dbo.t_func
   (
       @t_id int
   )
   RETURNS bit
   WITH SCHEMABINDING
   AS
   BEGIN
       DECLARE @r bit;
       IF EXISTS (SELECT 1 FROM dbo.t WHERE t.t_id = @t_id)
           SET @r = 1
       ELSE
           SET @r = 0;
       RETURN @r;
   END
   GO
   
   CREATE FUNCTION dbo.t_tvf
   (
       @min_t_id int
       , @max_t_id int
   )
   RETURNS TABLE 
   WITH SCHEMABINDING
   AS
   RETURN (
       SELECT t_id = t.t_id
           , e = dbo.t_func(dbo.t.t_id)
       FROM dbo.t
       WHERE t.t_id >= @min_t_id
           AND t.t_id <= @max_t_id
   );
   GO
   

3. Insert some sample data into the table:

   INSERT INTO dbo.t (t_id)
   SELECT ROW_NUMBER() OVER (ORDER BY c.id, c.colid)
   FROM sys.syscolumns c;
   GO
   

4. Create a table to store function execution stats, and populate it with a start-row showing execution counts for the multi-statement-function, t_func:

   CREATE TABLE dbo.function_stats
   (
       run_num int NOT NULL
       , object_name sysname NOT NULL
       , execution_count int NULL 
       , CONSTRAINT PK_function_stats
           PRIMARY KEY CLUSTERED (run_num, object_name)
   );
   GO
   INSERT INTO dbo.function_stats (run_num, object_name, execution_count)
   SELECT 1
       , o.name
       , COALESCE(fs.execution_count, 0)
   FROM sys.objects o 
       LEFT JOIN sys.dm_exec_function_stats fs ON fs.object_id = o.object_id
   WHERE o.name = 't_func';
   GO
   

5. Run a query against the TVF:

   SELECT t.*
   FROM dbo.t_tvf(1, 2) t;
   GO
   

6. Capture the execution stats now:

   INSERT INTO dbo.function_stats (run_num, object_name, execution_count)
   SELECT 2
       , o.name
       , COALESCE(fs.execution_count, 0)
   FROM sys.objects o 
       LEFT JOIN sys.dm_exec_function_stats fs ON fs.object_id = o.object_id
   WHERE o.name = 't_func';
   

7. The function stats results:

   SELECT *
   FROM dbo.function_stats fs
   ORDER BY fs.run_num
       , fs.object_name;
   

╔═════════╦═════════════╦═════════════════╗
║ run_num ║ object_name ║ execution_count ║
╠═════════╬═════════════╬═════════════════╣
║       1 ║ t_func      ║               0 ║
║       2 ║ t_func      ║               2 ║
╚═════════╩═════════════╩═════════════════╝

As you can see, the multi-statement-function has execute twice, once per row for the source table accessed by the TVF.

I expect the mutli-statement-function is being called many, many times by the TVF, giving the impression that it is running slowly, whereas in fact it is simply being called many times.