createIndex<http://www.w3.org/TR/IndexedDB/#widl-IDBObjectStore-createIndex-IDBIndex-DOMString-name-any-keyPath-IDBIndexParameters-optionalParameters>API specification state that "If keyPath is and Array and the multiEntry property in the optionalParameters is true, then a DOMException of type NotSupportedError must be thrown".
I believe NotSupportedError is unnecessary. multiEntry is no different than non-multiEntry index value, except the reference value is repeated. This specification limit generalizes usage of composite index for key joining algorithm. Google appengine datastore also have have multiEntry (ListProperty<https://developers.google.com/appengine/docs/python/datastore/typesandpropertyclasses#ListProperty>). It has no special different in indexing, other than limiting number of entries<https://groups.google.com/forum/?fromgroups=#!topic/google-appengine/1fTct9AO1MY>and warning for possibility of explosive index. Composite index with multiEntry is very useful, like modelling graph data and many-to-many relationship. Currently query on such model are limited to single index. It is also very unlikely that web developer will use excessive indexing. I propose NotSupportedError left out of specification. Best regards, Kyaw
