  Private information retrieval (PIR), a privacy-preserving cryptographic tool,
solves a simplified version of this problem by hiding the database item that a
client accesses. Most PIR protocols require the client to know the exact row
index of the intended database item, which cannot support the complicated
aggregation-based statistical query in a similar setting. Some works in the PIR
space contain keyword searching and SQL-like queries, but most need multiple
interactions between the PIR client and PIR servers. Some schemes support
searching SQL-like expressive queries in a single round but fail to enable
aggregate queries. These schemes are the main focus of this paper. To bridge
the gap, we have built a general-purpose novel information-theoretic PIR
(IT-PIR) framework that permits a user to fetch the aggregated result, hiding
all sensitive sections of the complex query from the hosting PIR server in a
single round of interaction. In other words, the server will not know which
records contribute to the aggregation. We then evaluate the feasibility of our
protocol for both benchmarking and real-world application settings. For
instance, in a complex aggregate query to the Twitter microblogging database of
1 million tweets, our protocol takes 0.014 seconds for a PIR server to generate
the result when the user is interested in one of 3K user handles. In contrast,
for a much-simplified task, not an aggregate but a positional query, Goldberg's
regular IT-PIR (Oakland 2007) takes 1.13 seconds. For all possible user
handles, 300K, it takes equal time compared to the regular IT-PIR. This example
shows that complicated aggregate queries through our framework do not incur
additional overhead if not less, compared to the conventional query.
