ClickHouse tips #6: Filtering data in subqueries to avoid joins
Imagine that you want to join two tables, and filter by a column that comes from the table in the right side of the join. On ClickHouse the query a bit different than what you’d do in other databases like Postgres, and it will result in a big performance improvement.
Let’s say one of the tables is this {% code-line %}events{% code-line-end %} table, with 100M rows:
And the other table is this {% code-line %}products{% code-line-end %} one, with ~2M rows
If you will always filter the result after making the join as in the query above, you don’t need to make a join at all. ClickHouse saves data column-by-column, so filtering by the values in a column is a very fast operation. If you rewrite the query as follows, it would be just as fast. And you wouldn’t have create a Join table for it:
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