https://www.velodb.io/blog/asof-join-benchmark-apache-doris-vs-clickhouse-and-duckdb
A stock trade prints at 10.250. The quote that explains its price arrived at 10.980. An equality join will never pair those two rows, because the timestamps differ by 270 milliseconds. The query you need says: for each trade, find the latest quote at or before the trade time, for the same ticker.
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https://www.velodb.io/blog/asof-join-benchmark-apache-doris-vs-clickhouse-and-duckdb