Interactive Grid: Automatic Position Number

Create a new hidden “POSITION” item. During startup select the current max position value into that item. Add a Dynamic Action doing the magic inside your Interactive Grid.

Hidden Item: P1_CURRENT_POSITION_VALUE
Settings > Value Protected: No

Computation or Before Header Process:

select
   nvl(max(position), 0)
into
   :P1_CURRENT_POSITION_VALUE
 from YOUR_TABLE
where id = :P1_ID;

Interactive Grid
Column Name: POSITION
Type: Number Field
This column must be the first column in your IG.

Dynamic Action Event: Row Initialization [Interactive Grid]
Selection Type: Region
Region: Your Interactive Grid Region

True Action: Set Value
Set Type: JavaScript Expression

( $(this.affectedElements[0]).val() ? $(this.affectedElements[0]).val() : $('#P1_CURRENT_POSITION_VALUE').val(parseInt($('#P1_CURRENT_POSITION_VALUE').val(),10)+1).val() )

Affected Elements
Selection Type: Column(s)
Column(s): POSITION