blockstream-electrs/doc/schema.md

Index Schema

The index is stored at a single RocksDB database using the following schema:

Transaction outputs' index

Allows efficiently finding all funding transactions for a specific address:

Code	Script Hash Prefix	Funding TxID Prefix
b'O'	SHA256(script)[:8]	txid[:8]

Transaction inputs' index

Allows efficiently finding spending transaction of a specific output:

Code	Funding TxID Prefix	Funding Output Index	Spending TxID Prefix
b'I'	txid[:8]	uint16	txid[:8]

Full Transaction IDs

In order to save storage space, we store the full transaction IDs once, and use their 8-byte prefixes for the indexes above.

Code	Transaction ID	Confirmed height
b'T'	txid (32 bytes)	uint32

Note that this mapping allows us to use getrawtransaction RPC to retrieve actual transaction data from without -txindex enabled (by explicitly specifying the blockhash).