Stored PSQL scalar functions are not supported in this version but they are coming in Firebird 3. In Firebird 2.5 and below, you can instead write a selectable stored procedure that returns a scalar result and SELECT it from your DML query or subquery.

Example

  |SELECT
  |  PSQL_FUNC(T.col1, T.col2) AS col3,
  |  col3
  |FROM T

can be replaced with:

  |SELECT
  |  (SELECT output_column FROM  PSQL_PROC(T.col1)) AS col3,
  |  col2
  |FROM T

or

  |SELECT
  |  output_column AS col3,
  |  col2,
  |FROM T
  |LEFT JOIN PSQL_PROC(T.col1)