A block explorer is typically made of:
an indexer that extracts the on-chain data and stores it into a database
an API that queries the database
a frontend that displays the data
So, the indexer is the part of the explorer that fetches the raw data from the node, then processes it and stores it in the database in an efficient way to provide quick access to the blockchain data.
Indexers are node operators. The ETL extract, transform and load data into the SQL database by mapping the data into a pre-defined schema of tables with referential integrity in order to provide indexing and query processing services via the API.
A Tezos Node is the heart of the blockchain, it manages the protocol.
ETL stands for extract, transform, and load The process of ETL plays a key role in data integration strategies. ETL allow businesses to gather data from multiple sources and consolidate it into a single, centralized location.
API is the acronym for Application Programming Interface, which is a software intermediary that allows two applications to talk to each other.
The Blockwatch indexer replaces the slow and expensive SQL datastore with a high-performance columnar database that allows for extremely fast analytical queries.
Columnar database is a column-oriented storage for database. It is optimized for fast retrieval of data columns, for example for analytical applications. It significantly reduces the overall disk I/O requirements and limits the amount of data you need to load from the disk.
It's a custom-made database for blockchain analytics. Avoiding the storage bottleneck allows for more complex data processing.
Storage bottleneck is a situation where the flow of data gets impaired or stopped completely due to bad performance or lack of resources.
State updates happen at each block, which means all the balance updates are always verified, and the indexer will follow chain reorganizations in real-time.