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by CHY872 1267 days ago
How does Clickhouse handle downstream aggregations? E.g...

  SELECT AVG(col) FROM (SELECT MAX(col2) AS col FROM table GROUP BY col3) t;
type query? Essentially, materialized views which are very unstable? Flink has its retract streams where rows can be semantically removed from the output and downstream query plans can understand deletes, but my expectation has been that these have bad worst case performance.
1 comments
zX41ZdbW 1267 days ago
It does not handle deletes / updates of the source table at all - it means the data of the materialized view become unaffected. Nothing sophisticated...

About the multi-level aggregate queries - the only way is to define a materialized view for the inner query and do calculations on top of it on the fly. So, the data for the inner query will be pre-aggregated, but nothing on top of that.

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