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ClickHouse reads table data in blocks of 'max_block_size' rows. Due to filtering, aggregation, etc., result blocks are typically much smaller than 'max_block_size' but there are also cases where they are much bigger. Setting 'query_cache_squash_partial_results' (enabled by default) now controls if result blocks are squashed (if they are tiny) or split (if they are large) into blocks of 'max_block_size' size before insertion into the query result cache. This reduces performance of writes into the query cache but improves compressability of cache entries and provides more natural block granularity when query results are later served from the query cache. Entries in the query cache are now also compressed by default. This reduces the overall memory consumption at the cost of slower writes into / reads from the query cache. To disable compression, use setting 'query_cache_compress_entries'. |
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