docstore/all: add Offset method, useful for pagination (#3385)

docstore/awsdynamodb/query.go

@@ -53,9 +53,10 @@ func (c *collection) RunGetQuery(ctx context.Context, q *driver.Query) (driver.D
 		return nil, err
 	}
 	it := &documentIterator{
-		qr:    qr,
-		limit: q.Limit,
-		count: 0, // manually count limit since dynamodb uses "limit" as scan limit before filtering
+		qr:     qr,
+		offset: q.Offset,
+		limit:  q.Limit,
+		count:  0, // manually count limit since dynamodb uses "limit" as scan limit before filtering
 	}
 	it.items, it.last, it.asFunc, err = it.qr.run(ctx, nil)
 	if err != nil {
@@ -458,17 +459,25 @@ func toInCondition(f driver.Filter) expression.ConditionBuilder {
 }
 
 type documentIterator struct {
-	qr     *queryRunner
-	items  []map[string]*dyn.AttributeValue
-	curr   int
-	limit  int
-	count  int // number of items returned
-	last   map[string]*dyn.AttributeValue
-	asFunc func(i interface{}) bool
+	qr     *queryRunner                     // the query runner
+	items  []map[string]*dyn.AttributeValue // items from the last query
+	curr   int                              // index of the current item in items
+	offset int                              // number of items to skip
+	limit  int                              // number of items to return
+	count  int                              // number of items returned
+	last   map[string]*dyn.AttributeValue   // lastEvaluatedKey from the last query
+	asFunc func(i interface{}) bool         // for As
 }
 
 func (it *documentIterator) Next(ctx context.Context, doc driver.Document) error {
-	if it.limit > 0 && it.count >= it.limit || it.curr >= len(it.items) && it.last == nil {
+	// Skip the first 'n' documents where 'n' is the offset.
+	if it.offset > 0 && it.count < it.offset {
+		it.curr++
+		it.count++
+		return it.Next(ctx, doc)
+	}
+	// Only start counting towards the limit after the offset has been reached.
+	if it.limit > 0 && it.count >= it.offset+it.limit || it.curr >= len(it.items) && it.last == nil {
 		return io.EOF
 	}
 	if it.curr >= len(it.items) {