A non-relational data store may implement validating and non-validating secondary indexes for a table. Operations at a table for a given item may be performed when indexing the item to create a secondary index or when updates to the given item are received. Attribute values of a given item may be validated with respect to an indexing schema for the secondary index. For a non-validating secondary index, validation errors detected for the attribute values may be ignored so that the operation at the table may be performed. For a validating secondary index, validation errors detected for the attribute values may result in denying performance of the operation. In some embodiments, a secondary index from may be changed from validating to non-validating, or non-validating to validating.