Correct severe misconception about SetCurrentKey in the knowledge base

microsoft/knowledge/performance/apply-filters-before-iterating.md

@@ -15,7 +15,7 @@ A `SetRange` or `SetFilter` placed before `FindSet` narrows the result set at th
 
 ## Best Practice
 
-Move every predicate that can be expressed as an equality or range filter into a `SetRange` or `SetFilter` ahead of the find. Combine with `SetCurrentKey` to choose a key whose first fields match the filter (see `setcurrentkey-aligns-key-with-filters.md`). The loop body should then contain only the work that depends on per-row state.
+Move every predicate that can be expressed as an equality or range filter into a `SetRange` or `SetFilter` ahead of the find. Make sure a key (index) exists whose leading fields cover the filter so the optimizer can seek; note that `SetCurrentKey` only sets sort order and is not an index hint (see `setcurrentkey-sets-sort-order-not-index-hint.md`). The loop body should then contain only the work that depends on per-row state.
 
 See sample: `apply-filters-before-iterating.good.al`.
 

microsoft/knowledge/performance/setcurrentkey-sets-sort-order-not-index-hint.bad.al

@@ -0,0 +1,20 @@
+codeunit 50231 "Perf Sample SetCurrentKey Bad"
+{
+    // Misconception: SetCurrentKey does NOT tell SQL Server to use this index.
+    // The optimizer picks the index from the filters (WHERE clause) and statistics.
+    // The result order is never used here, so SetCurrentKey only adds an ORDER BY
+    // the query does not need — and can push the plan toward a sort.
+    procedure SumRemainingAmount(CustomerNo: Code[20]) Total: Decimal
+    var
+        CustLedgerEntry: Record "Cust. Ledger Entry";
+    begin
+        CustLedgerEntry.SetCurrentKey("Customer No.", Open, "Posting Date");
+        CustLedgerEntry.SetRange("Customer No.", CustomerNo);
+        CustLedgerEntry.SetRange(Open, true);
+        CustLedgerEntry.SetAutoCalcFields("Remaining Amount");
+        if CustLedgerEntry.FindSet() then
+            repeat
+                Total += CustLedgerEntry."Remaining Amount";
+            until CustLedgerEntry.Next() = 0;
+    end;
+}

microsoft/knowledge/performance/setcurrentkey-sets-sort-order-not-index-hint.good.al

@@ -0,0 +1,18 @@
+codeunit 50230 "Perf Sample SetCurrentKey Good"
+{
+    // SetCurrentKey is used because the rows must be processed oldest-first.
+    // The sort is a functional requirement, so the ORDER BY it adds is justified.
+    procedure ApplyOldestEntriesFirst(CustomerNo: Code[20])
+    var
+        CustLedgerEntry: Record "Cust. Ledger Entry";
+    begin
+        CustLedgerEntry.SetRange("Customer No.", CustomerNo);
+        CustLedgerEntry.SetRange(Open, true);
+        CustLedgerEntry.SetCurrentKey("Posting Date");
+        if CustLedgerEntry.FindSet() then
+            repeat
+                // Apply entries in posting-date order ...
+            until CustLedgerEntry.Next() = 0;
+    end;
+}
+

microsoft/knowledge/performance/setcurrentkey-sets-sort-order-not-index-hint.md

@@ -0,0 +1,35 @@
+---
+bc-version: [all]
+domain: performance
+keywords: [setcurrentkey, sort, order-by, index, key, query-optimizer, hint]
+technologies: [al]
+countries: [w1]
+application-area: [all]
+---
+
+# SetCurrentKey only sets sort order — it is not an index hint
+
+## Description
+
+A common misconception is that `SetCurrentKey` tells SQL Server which index to use for a query. It does not. In Business Central, `SetCurrentKey` only changes the `ORDER BY` clause of the generated SQL statement. It does not add an index hint, and the SQL Server query optimizer is free to ignore the named key entirely.
+
+The optimizer picks the index from the `WHERE` clause (your `SetRange`/`SetFilter`) together with table statistics and estimated cost. In practice it almost never chooses an index just because that key appears in `ORDER BY`. So calling `SetCurrentKey` to "steer" the plan toward an index is a no-op for index selection — and can make things worse: an `ORDER BY` that the query does not otherwise need can push the optimizer toward a less selective index or add a Sort operator to the plan.
+
+Selectivity comes from having the right index available (a key on the table whose leading fields cover the filter) and from filtering on those fields — not from `SetCurrentKey`.
+
+## Best Practice
+
+Decide `SetCurrentKey` on one question only: **do I need the result set in a specific order?**
+
+- If yes — you iterate rows in a defined sequence, or rely on `FindFirst`/`FindLast`/`Next` returning a particular row — call `SetCurrentKey` for that sort. The order is a functional requirement, and the `ORDER BY` is justified.
+- If no — omit `SetCurrentKey`. Let the optimizer choose the cheapest plan for your filters; it may pick a better index and skip a sort.
+
+To make a filtered read fast, ensure a key (index) exists on the table whose leading fields cover the filter, and filter on those fields with `SetRange`/`SetFilter`. That is what lets the optimizer seek. Defining the key creates the index; `SetCurrentKey` is not required to make the optimizer use it.
+
+See sample: `setcurrentkey-sets-sort-order-not-index-hint.good.al`.
+
+## Anti Pattern
+
+Adding `SetCurrentKey` to a filtered read purely in the belief that it forces SQL Server to seek a particular index, when the code never uses the resulting order. This does nothing for index selection and only appends an `ORDER BY` the query does not need, risking an unnecessary sort. Remove the `SetCurrentKey`; rely on the filters and an existing covering key instead.
+
+See sample: `setcurrentkey-sets-sort-order-not-index-hint.bad.al`.