Indexing an application provides more efficient access to its organizational information. For example, search engines index the entire Internet, and you can search for information more easily through it
Indexing blockchain data is really, really hard.Blockchain properties like finality, chain reorganizations, or uncled blocks complicate this process further, and make it not just time consuming but conceptually hard to retrieve correct query results from blockchain data.
In addition, developers can build APIs (indexed "subgraphs") can used to query data specific to a set of smart contracts. Data is fetched with a standard GraphQL API. You can visit The Graph's documentation site to read more about The Graph protocol.
Due to the support of Ethereum tracing modules in REI network, The Graph is capable of indexing blockchain data in REI. This guide takes you through the creation of a simple subgraph for a Bank contract on testnet. This guide can be adapted for mainnet.