Currently I am working in a project that use PostgreSQL + ElasticSearch. However I recently found VoltDB, and I was wondering if we still need ElasticSearch for doing searches with VoltDB.
If I am ok, elasticSearch get the data from PostgreSQL of from another relational Database, and them it reindexes the data to make faster Queries instead using the relational Database indexes. This is because the data stored in ElasticSearch is not completely trusted because ElasticSearch is not ACID compliant.
Based on my (limited) knowledge of ElasticSearch, it appears that it is a search server that would work in conjunction with the database and is used primarily to search and index document files.
If this is correct, I don't think that NuoDB would be a replacement for ElasticSearch but could likely work in conjunciton with it similar to PostgreSQL.
Also, similar to Volt, NuoDB doesn't have full-text-indexing functionality.