Query cache: Enable compression of cache entries#45912
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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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See #48358 (comment). |
rschu1ze
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Apr 3, 2023
Failing with high rate in master after #45912 was merged
devcrafter
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Apr 3, 2023
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| -- Create test table with lot's of rows | ||
| CREATE TABLE t(c String) ENGINE=MergeTree ORDER BY c; | ||
| INSERT INTO t values ('abc') ('def') ('abc') ('jkl'); |
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INSERT INTO t SELECT multiIf(n = 0, 'abc', n = 1, 'def', n = 2, 'abc', 'jkl')
FROM
(
SELECT number % 4 AS n
FROM numbers(4 * 400)
)
rschu1ze
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- clear query cache at the end of the tests to minimize interaction with other query cache tests - generate data more elegantly
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This PR changes two things in the query cache:
ClickHouse reads table data in blocks of
max_block_sizerows but due to filtering, aggregation, etc., result blocks are typically much smaller thanmax_block_size(and sometimes bigger). The new settingquery_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 ofmax_block_sizesize before insertion into the query result cache. This reduces performance of writes into the query cache but improves compressability of cache entries and provides a "natural" block granularity when query results are later served from the query cache.Entries in the query cache are now 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 the new setting
query_cache_compress_entries.Changelog category (leave one):
Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):
Entries in the query cache are now squashed to max_block_size and compressed