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geysersam04/23/20251 replyview on HN

Sorry if this question exposes my naivety, why such a low default limit? What drawback does lazy materialization have that makes it good to have such a low limit?

Do you know any example query where lazy materialization is detrimental to performance?


Replies

nasretdinov04/23/2025

My understanding is that with higher limit values you may end up doing lots of random I/O (for each granule the order in which you read it would be much less predictable than when ClickHouse normally reads it sequentially), essentially one I/O operation per LIMIT value. So larger default values would only be beneficial in pathological examples given in the article, but much less so in "real world".