· Jan 14, 2018

Find last in a list of objects

I have a class to track data changes.
As it becomes slower and slower to find last changes I think I need an index.
The 2 classes are just simplified reality.
HowTo ?

Class Rick.ChangeItem Extends %SerialObject
Property Subject As %String;

Property Author As %String;

Property Change As %TimeStamp;


Class DC.listItem Extends (%Persistent)
Property Title As %String;

Property Changes As List Of Rick.ChangeItem ;

/// where & how create the index ?
Discussion (6)0
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I see 2 options


add the ChangeItem holding the last change in addition to your list of Objects.
and add the appiopriate indices.
like this:

Class Rick.ChangeList Extends %Persistent
  Property Title As %String;
  Property Changes As list Of Rick.ChangeItem;
/// Last change added to list
  Property Last As Rick.ChangeItem;
  Index xlast On Last.Change;


This looks simple but it requires that any program that adds a change also 
updates the "Last" property.
If this isn't feasible you may use %OnBeforeAddToSaveSet() or similar to insert you "Last" object during %Save().
It requires more space on disk and is not redundant.
It doesn't generate entries for existent records.This requires a special update exercise.


Add the property to be indexed as SqlComputed and index it.
This is not so speedy but gives you the flexibility not just check the last element
and has no impact or dependency on any existing code using it.

Class Rick.ChangeList Extends %Persistent
  Property Title As %String;
  Property Changes As list Of Rick.ChangeItem;
  Property lastDate As %TimeStamp [ Calculated, SqlComputed ,
          SqlComputeCode = { set {*} = ##class(Rick.ChangeList).ChangeDate({ID}) }];

Index xdate On lastDate [ Data = lastText ];
ClassMethod ChangeDate(ID As %Integer)
  set obj=##class(Rick.ChangeList).%OpenId(ID)    
   quit item.Change

When talking about speed, one of the most important questions is what exactly do you need to speed up? You need to determine:

  • what queries are the most popular
  • what queries take the most time

to fix it. Some ways to fix performance are (easy to hard):

  • Add indices
  • Change classes
  • Change application architecture

Because in your case I see at least two request types which require different actions to speed them up:

  • Get last X changes
  • Get last X modified subjects