Blockchains’ power of decentralization, immutability, and
transparency has found its application in many fields. The blockchain
is an append-only structure. As the blockchain grows, accessing blocks
from the past in an efficient manner has become a challenging task.
Further, blockchains were not initially envisioned to be read-heavy
systems. With the passage of time, more and more applications are
using blockchains and therefore blockchains need to provide active
support for high read loads concerning the history state as well. To
facilitate data querying in blockchain, we have proposed an SQL query
processing feature in the Ethereum blockchain through a decentralized
application. More specifically, we build an Ethereum-based Electronic
Health Record (EHR) system with SQL query support. The following
approaches for query processing have been explored and implemented:
(i) linearly scanning blockchain, (ii) scanning only from a user-
specified block, (iii) replication in database, (iv) indexing in a database,
and (v) using smart contracts. Our timing analysis across these
implementations reveals that the smart contracts-based approach has
reasonable performance gains compared the other approaches.