sql/rowexec: ddl_iters.go: PR feedback.

Author: reltuk

sql/rowexec/ddl_iters.go

@@ -1855,7 +1855,6 @@ func (b *BaseBuilder) executeCreateCheck(ctx *sql.Context, c *plan.CreateCheck)
 	if err != nil {
 		return err
 	}
-	rowIter = withSafepointPeriodicallyIter(rowIter)
 
 	for {
 		row, err := rowIter.Next(ctx)

sql/rowexec/dml_iters.go

@@ -638,6 +638,19 @@ func withSafepointPeriodicallyIter(child sql.RowIter) *safepointPeriodicallyIter
 	return &safepointPeriodicallyIter{child: child}
 }
 
+// A wrapper iterator which calls sql.SessionCommandSafepoint on the
+// ctx.Session periodically while returning rows through calls to
+// |Next|.
+//
+// Should be used to wrap any iterators which are involved in
+// long-running write operations and which are exhausted or iterated
+// by other iterators in the iterator tree, such as accumulatorIter.
+//
+// This iterator makes the assumption that a safepoint, from the
+// Engine's perspective, can be established at any moment we are
+// within a Next() call. This is generally true given the Engine's
+// lack of concurrency on a given Session, but if something like
+// Exchange node came back, this would not necessarily be true.
 type safepointPeriodicallyIter struct {
 	child sql.RowIter
 	n     int