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Correct performance knowledge guidance

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microsoft/knowledge/performance/prefer-modifyall-over-per-row-modify.md
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@ -7,20 +7,20 @@ countries: [w1]
application-area: [all]
---
# Use ModifyAll / DeleteAll instead of per-row Modify / Delete in a loop
# Use ModifyAll only for equivalent bulk assignments
## Description
`ModifyAll` and `DeleteAll` are the bulk APIs. Per the upstream guidance, they "execute as single SQL statements" when the table supports it — one round-trip updates or deletes every row in the filtered set. The anti-pattern is the loop equivalent: `FindSet` followed by per-row `Modify`/`Delete`, where the runtime issues one write per row. On a production-scale table the difference is the difference between a single statement and N statements.
`ModifyAll` assigns one value to one field across the filtered set. It does not run the field's `OnValidate` trigger. Its optional `RunTrigger` parameter controls the table `OnModify` trigger, not field validation. Replacing a loop is therefore correct only when direct assignment is semantically equivalent for every row.
## Best Practice
When the loop body does nothing more than assign a constant value (or a value computed once) to one or more fields, replace the loop with `ModifyAll("Field 1", Value1)` — and chain additional `ModifyAll` calls for additional fields. The same shape applies to `DeleteAll`. Be aware that the bulk APIs can regress to row-by-row execution for tables with certain trigger or media-field configurations (see `triggers-and-media-field-regress-modifyall.md`); when that regression applies, multiple `ModifyAll` calls become more expensive than one manual loop, so the choice is conditional, not absolute.
Use `ModifyAll` when the loop directly assigns the same value, does not call `Validate`, needs no per-row calculation, and does not depend on `OnModify` unless the equivalent `RunTrigger` value is supplied. Check whether table-extension triggers, event subscribers, global triggers, or media fields force row-by-row fallback (see `triggers-and-media-field-regress-modifyall.md`).
See sample: `prefer-modifyall-over-per-row-modify.good.al`.
## Anti Pattern
`if SalesLine.FindSet() then repeat SalesLine.Validate("Unit Price", NewPrice); SalesLine.Modify(true); until SalesLine.Next() = 0;` — N writes when one would do. The pattern is easy to introduce when the loop initially does per-row computation and is later simplified to assign a constant; the loop scaffolding survives the simplification.
A loop that only assigns a constant and calls `Modify(false)` on a field with no validation side effects. Conversely, replacing `Validate(Field, Value); Modify(true)` with `ModifyAll(Field, Value)` is also an anti-pattern because it silently drops field validation and may drop table-trigger behavior.
See sample: `prefer-modifyall-over-per-row-modify.bad.al`.