rowexec: fix usage of the shared "single datum" acc in windower

This commit fixes the usage of the shared "single datum agg mem account"
by the window builtins. Previously, we were updating the eval context
with the memory account after the builtins were constructed. The impact
of the bug on its own is pretty minor (namely that we could reserve more
memory than necessary - the initial allocation is 10KiB), but I have
some other changes brewing which make it more important to be precise
about the attribution of the memory allocations.

Release note: None

pkg/sql/rowexec/windower.go

@@ -112,6 +112,26 @@ func newWindower(
 	w.inputTypes = input.OutputTypes()
 	ctx := evalCtx.Ctx()
 
+	// Limit the memory use by creating a child monitor with a hard limit.
+	// windower will overflow to disk if this limit is not enough.
+	limit := execinfra.GetWorkMemLimit(flowCtx)
+	if limit < memRequiredByWindower {
+		// The limit is set very low (likely by the tests in order to improve
+		// the test coverage), but the windower requires some amount of RAM, so
+		// we override the limit. This behavior is acceptable given that we
+		// don't expect anyone to lower the setting to less than 100KiB in
+		// production.
+		limit = memRequiredByWindower
+	}
+	limitedMon := mon.NewMonitorInheritWithLimit("windower-limited", limit, evalCtx.Mon)
+	limitedMon.StartNoReserved(ctx, evalCtx.Mon)
+	w.acc = limitedMon.MakeBoundAccount()
+	// If we have aggregate builtins that aggregate a single datum, we want
+	// them to reuse the same shared memory account with the windower. Notably,
+	// we need to update the eval context before constructing the window
+	// builtins.
+	evalCtx.SingleDatumAggMemAccount = &w.acc
+
 	w.partitionBy = spec.PartitionBy
 	windowFns := spec.WindowFns
 	w.windowFns = make([]*windowFunc, 0, len(windowFns))
@@ -145,20 +165,6 @@ func newWindower(
 	}
 	w.outputRow = make(rowenc.EncDatumRow, len(w.outputTypes))
 
-	// Limit the memory use by creating a child monitor with a hard limit.
-	// windower will overflow to disk if this limit is not enough.
-	limit := execinfra.GetWorkMemLimit(flowCtx)
-	if limit < memRequiredByWindower {
-		// The limit is set very low (likely by the tests in order to improve
-		// the test coverage), but the windower requires some amount of RAM, so
-		// we override the limit. This behavior is acceptable given that we
-		// don't expect anyone to lower the setting to less than 100KiB in
-		// production.
-		limit = memRequiredByWindower
-	}
-	limitedMon := mon.NewMonitorInheritWithLimit("windower-limited", limit, evalCtx.Mon)
-	limitedMon.StartNoReserved(ctx, evalCtx.Mon)
-
 	if err := w.InitWithEvalCtx(
 		w,
 		post,
@@ -191,11 +197,6 @@ func newWindower(
 		return nil, err
 	}
 
-	w.acc = w.MemMonitor.MakeBoundAccount()
-	// If we have aggregate builtins that aggregate a single datum, we want
-	// them to reuse the same shared memory account with the windower.
-	evalCtx.SingleDatumAggMemAccount = &w.acc
-
 	if execstats.ShouldCollectStats(ctx, flowCtx.CollectStats) {
 		w.input = newInputStatCollector(w.input)
 		w.ExecStatsForTrace = w.execStatsForTrace