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Источник: http://axcoder.blogspot.com/2007/07/...selecting.html
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Last friday i was optimizing a form with a lof of display fields with an implementation like the following:

display EmplName emplName()
{
  return EmplTable::find(this.EmplID).Name;
}

The one of sources to increase performance of such code is to select only fields which will be used lately. So I have replaced this code with the following:

display EmplName emplName()
{
  return this.EmplID ?
         (select Name from EmplTable
              where EmplTable.emplID==this.EmplID).Name
         :
         "";
}

but when I enabled SQL tracing, I found, that this code producess select with ALL fields of EmplTable.
It was strange... With a great help of my co worker, i've found the following:

• when there is only fields of any unique index in the where condition, all fields are fetched
• when there is an extra field in the condition, only required fields is selected (and the extra fields in where)
• So I converted previously mentioned methods to the following:

display EmplName emplName()
{
  return this.EmplID ?
         (select Name from EmplTable
              where EmplTable.emplID==this.EmplID
                     &&
                     EmplTable.recID
          ).Name
         :
         "";
}

Источник: http://axcoder.blogspot.com/2007/07/...selecting.html