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Correct performance knowledge guidance (#94)

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Co-authored-by: Jesper Schulz-Wedde <jesper.schulzwedde@microsoft.com>
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microsoft/knowledge/performance/addloadfields-in-report-onpredataitem.md
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@ -7,20 +7,20 @@ countries: [w1]
application-area: [all]
---
# In reports, declare the fields the layout needs with AddLoadFields
# Add trigger-only report fields in OnPreDataItem
## Description
Reports iterate dataitems on potentially large source tables and pipe rows into a layout. The partial-record optimization is the same idea as `use-setloadfields-for-partial-records.md`, but the API is different: per the upstream guidance, "for reports, use `AddLoadFields()` in `OnPreDataItem` trigger to add fields needed by the layout." `AddLoadFields` is additive — call it for each field the layout consumes — and runs once per dataitem before iteration begins.
Report dataitem field selection is calculated at compile time and once per dataitem type during execution. Fields referenced by dataset columns are selected automatically; fields used only in triggers are not. Use `AddLoadFields` in `OnPreDataItem` to supplement the automatic selection with normal fields that trigger code needs.
## Best Practice
In each dataitem's `OnPreDataItem` trigger, list the columns the layout binds to via `AddLoadFields(<field>, <field>, ...)`. The platform then materializes only those columns per row. Treat the layout column list as the spec: every column the layout uses must be added; columns the layout does not use should not be added.
When a dataitem trigger needs an extra field, add that field in `OnPreDataItem` before iteration starts. This supplements the compiler-selected fields and avoids the first just-in-time load and enumerator update when the trigger reads the extra field.
See sample: `addloadfields-in-report-onpredataitem.good.al`.
## Anti Pattern
Relying on the dataitem's default to load every field. On a report bound to a ledger-scale table this transfers an entire row per iteration, of which the layout reads a fraction.
Listing every dataset column in `AddLoadFields`, or omitting a known trigger-only field because the dataset already uses other fields. The former is redundant; the latter causes a just-in-time load on first access and can cause repeated loads when the record is copied or passed by value.
See sample: `addloadfields-in-report-onpredataitem.bad.al`.