Question

recently, while working on a db2 -> oracle migration project, we came across this situation.
the developers were inadvertently creating new table structures using decimal(s,p) columns. I didn’t remember Oracle supporting this, but then some digging showed that its a ANSI data type therefore supported by oracle.

However, question for me remained –

1. how is this data handled internally ?
2. is there a cost of using ANSI types instead of Oracle’s built in types ?
3. Will there be an impact during the data migration if the target type was Oracle built-in type ?

Answer 1

In Oracle, they are the same:

SQL statements that create tables and clusters can also use ANSI data
types and data types from the IBM products SQL/DS and DB2. Oracle
recognizes the ANSI or IBM data type name that differs from the Oracle
Database data type name. It converts the data type to the equivalent
Oracle data type, records the Oracle data type as the name of the
column data type, and stores the column data in the Oracle data type
based on the conversions shown in the tables that follow.

The table below this quote shows that DECIMAL(p,s) is treated internally as a NUMBER(p,s):

SQL> create table t (a decimal(*,5), b number (*, 5));

Table created

SQL> desc t;
Name Type        Nullable Default Comments 
---- ----------- -------- ------- -------- 
A    NUMBER(*,5) Y                         
B    NUMBER(*,5) Y  

However, the scale defaults to 0 for DECIMAL, which means that DECIMAL(*) is treated as NUMBER(*, 0), i.e. INTEGER:

SQL> create table t (a decimal, b number, c decimal (5), d decimal (5));

Table created

SQL> desc t;
Name Type      Nullable Default Comments 
---- --------- -------- ------- -------- 
A    INTEGER   Y                         
B    NUMBER    Y                         
C    NUMBER(5) Y                         
D    NUMBER(5) Y