Amazon DocumentDB, with MongoDB compatibility
My bet is that it is built on top of Aurora PostgreSQL. By looking at the "Limits" section (<a href="https://docs.aws.amazon.com/documentdb/latest/developerguide/limits.html" rel="nofollow">https://docs.aws.amazon.com/documentdb/latest/developerguide...</a>), identifiers are limited to 63 characters and the same characters that PostgreSQL limits identifiers to; and a collection size limit of 32TB, coincidentally maximum PostgreSQL table size.<p>Edit:  I can confirm: does not allow the UTF-8 null character in strings: <a href="https://docs.aws.amazon.com/documentdb/latest/developerguide/functional-differences.html" rel="nofollow">https://docs.aws.amazon.com/documentdb/latest/developerguide...</a> ... It is written on top of PostgreSQL.