Merge cockroachdb#84652

84652: workload/kv: randomly choose the start key for limited scans r=arulajmani a=arulajmani

The `span-limit` (in conjunction with `span-percent`) is used to
configure the kv workload to perform limited scans. Previously, we
would always perform these scans at the start of the keyspace. With this
patch, we now start the scan at a random start key instead. The key is
selected randomly based on the key generator specified for regular
read/write ops (uniform, sequential, or zipfian).

The behaviour of `span-limit` 0 is left unchanged.

Release note: None

Co-authored-by: Arul Ajmani <[email protected]>

pkg/workload/kv/kv.go

@@ -333,7 +333,11 @@ func (w *kv) Ops(
 	buf.Reset()
 	buf.WriteString(`SELECT count(v) FROM [SELECT v FROM kv`)
 	if w.spanLimit > 0 {
-		fmt.Fprintf(&buf, ` ORDER BY k LIMIT %d`, w.spanLimit)
+		// Span statements without a limit query all ranges. However, if there's
+		// a span limit specified, we want to randomly choose the range from which
+		// the limited scan starts at. We do this by introducing the k >= $1
+		// predicate.
+		fmt.Fprintf(&buf, ` WHERE k >= $1 ORDER BY k LIMIT %d`, w.spanLimit)
 	}
 	buf.WriteString(`]`)
 	spanStmtStr := buf.String()
@@ -411,7 +415,13 @@ func (o *kvOp) run(ctx context.Context) (retErr error) {
 	statementProbability -= o.config.readPercent
 	if statementProbability < o.config.spanPercent {
 		start := timeutil.Now()
-		_, err := o.spanStmt.Exec(ctx)
+		var err error
+		if o.config.spanLimit > 0 {
+			arg := o.g.readKey()
+			_, err = o.spanStmt.Exec(ctx, arg)
+		} else {
+			_, err = o.spanStmt.Exec(ctx)
+		}
 		elapsed := timeutil.Since(start)
 		o.hists.Get(`span`).Record(elapsed)
 		return err