$ORDER vs counting to scan global range - performance

Question

I have a choice between two ways of scanning through a key level in a large global array and am trying to figure out if one method is more efficient than the other.
This is a vendor supplied application and database on the Intersystems Caché database platform. It is written in the old MUMPS style and does not use any of Caché's object persistence functions: all data is stored in globals directly and any indexes are application maintained.
There is a common convention for repeating data elements attached to entities where the first record will contain a count of child records and then each child record is numbered sequentially at the next key level. For example:
^GBDATA(12345,100)="3"
^GBDATA(12345,100,1)="A^Record"
^GBDATA(12345,100,2)="B^Record"
^GBDATA(12345,100,3)="C^Record"
Where "12345" is the entity key, and "100" is one of the attached detail types. Note that the first "100" record with no other keys has the count of subrecords. There could be anywhere between 0 and hundreds of subrecords attached. The entities are often very wide and there is a lot of other data besides this subrecord type (not shown in example).
Given an entity key, I want to scan through all the subrecords of one type. Would it be faster to use $ORDER to go through the subkeys or to use a FOR loop to anticipate the key values? Does it matter?
$ORDER method:
SET EKEY=12345
SET SEQ=""
FOR
{
SET SEQ=$ORDER(^GBDATA(EKEY,100,SEQ), 1, ROWDATA)
QUIT:SEQ=""
WRITE ROWDATA,!
}
FOR count method:
SET EKEY=12345
SET LIM=^GBDATA(EKEY,100)
FOR SEQ=1:1:LIM
{
WRITE ^GBDATA(EKEY,100,SEQ),!
}
Does anyone know how $ORDER vs $GET is implemented internally in Caché?
I'm having trouble testing this empirically since we only have one production instance with appropriate data and I can't take it offline to clear the cache. I'm most interested in from-disk performance as opposed to from-cache performance.

Answer 1

You could use %SYS.MONLBL to figure out definitively. My guess is that $ORDER is slightly better.
http://docs.intersystems.com/cache20122/csp/docbook/DocBook.UI.Page.cls?KEY=GCM_monlbl

Answer 2

In regards to your question, "Does anyone know how $ORDER vs $GET is implemented internally in Caché?" The two are completely different functions.
$Order is used for the direction that you're going in when reviewing your ^Global.
$Get is used to pull the data within the ^Global. Below is an example of it's use. I use Cache ObjectScript; however, this should give you a general idea
Global Structure
^People(LastName,FirstName)="Phone"
Global Data
^People(Doe,John)="1035001234"
^People(Smith,Jane)="7405241305"
^People(Wood,Edgar)="7555127598"
Code Sample
SET LASTNAME=0
FOR QUIT:LASTNAME?." " DO
.SET LASTNAME=$ORDER(^People(LASTNAME)) QUIT:LASTNAME?." "
.SET FIRSTNAME=0
.FOR QUIT:FIRSTNAME?." " DO
..SET FIRSTNAME=$ORDER(^People(LASTNAME,FIRSTNAME)) QUIT:FIRSTNAME?." "
..SET PHONE=$GET(^People(LASTNAME,FIRSTNAME))
In the sample provided above, it will start with the first record within the ^People global and then start with the first record within the last name by utilizing $Order. It will then $Get the data for the ^People(LASTNAME,FIRSTNAME) node, which is the phone number.
For some samples and reference areas, check out the following links:
$Get Information
$Order Information