# An Efficient Blockchain Transaction Retrieval System
## 1 Introduction  
Blockchain technology originated from the Bitcoin project [1] invented by [PERSON]. This novel decentralized ledger technology has been used and explored in many fields, such as finance [2], digital copyright [3], supply chain management [4], and edge computing [5], and has achieved remarkable results. With the rapid development of blockchain, the blockchain has also exposed some problems. Because the blockchain data have the append-only characteristic, under continuous accumulation, they cause enormous storage pressure to the blockchain nodes [6]. According to the data analysis provided by the blockchain explore website, there are currently 670,000 blocks in Bitcoin, a total amount of data in the ledger close to 400 GB, involving 40 million account addresses. Ethereum has 12.24 million blocks. The total amount of data in the ledger has exceeded 1 TB, affecting 55 million account addresses [1, 7]. The impact of a large blockchain ledger is as follows [6]. Each blockchain node needs to store a complete copy of the blockchain ledger data, and the amount of ledger data puts too much pressure on the node's storage. At the same time, thousands of blockchain nodes all need to store ledger data, which can lead to a huge waste of storage resources. Too much pressure on a single point of storage can constrain new blockchain nodes from joining the blockchain network, leading to the centralization of the blockchain system,destroying the decentralized nature of the blockchain and directly affecting the scaling of the blockchain system. In response to the storage problems arising from blockchain nodes, academics have proposed storing blockchain data in third-party decentralized storage networks, such as IPFS, to reduce the storage pressure [8; 9; 10; 11]. The core idea of these blockchain storage solutions is to store the entire block data file to the IPFS network in order to reduce the storage strain on the blockchain nodes. The blockchain node is only responsible for storing the block hash and a summary of the transaction records. For acquisition of the block from the storage network, the blockchain node will utilize this hash and summary to verify the block data, ensuring that the blockchain is tamper proof. However, blockchain is a ledger, made up of a combination of blocks, which contains a portion of the ledger's transaction records [12; 13]. The transaction record is the smallest unit of the ledger, and the current blockchain transaction can support digital currency transfer, voucher record, logistics traceability and other kinds of information recording [14]. When the ledger data are stored in a third-party storage network, it raises the blockchain transaction retrieval problems. The blockchain transaction retrieval problem is because blockchain nodes no longer have the complete blockchain ledger data when storing the blockchain ledger in third-party storage networks [15; 16]. When blockchain ledgers are stored in third-party storage networks, blockchain nodes no longer have the entire blockchain ledger locally. Consequently, retrieving blockchain transactions becomes problematic. When users attempt to access the target transaction record, the blockchain node cannot locate the target block containing the target transaction. It requires blockchain nodes to regularly obtain the entire blockchain ledger from the storage network in order to access the transaction, resulting in considerable communication costs and poor user experience. Therefore, how to retrieve transaction in a large number of blocks based on the transaction records features becomes the most important aspect of the blockchain transaction retrieval challenge. The traditional blockchain transaction retrieval approach [1; 6] acquires the target transaction by traversing the blockchain ledger in local database by transaction hash, which is inefficient and cannot meet the requirements of transaction retrieval in many blocks in this scenario. Blockchain retrieval is a novel problem, and academia has also conducted extensive research on this.