Support to set max cache size for per query in local cache#37859
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kssenii merged 28 commits intoClickHouse:masterfrom Jun 10, 2022
Merged
Support to set max cache size for per query in local cache#37859kssenii merged 28 commits intoClickHouse:masterfrom
kssenii merged 28 commits intoClickHouse:masterfrom
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Description
Limiting the maximum cache usage per query can effectively prevent cache pool contamination.
Related Issues
Improvement
In the figure below, we perform several small queries (SSB's Q1.1), and after the third execution we run a large query (SSB's Q2.1, which scans almost the entire table). Running large query without limiting the maximum cache usage would pollute the entire cache pool, so continuing to run Q1.1 would require re-reading data from remote S3, causing performance fluctuations.
Use and Configuration
The user can set the maximum cache usage per query with
set max_query_cache_sizebefore running a query.The user can set the maximum cache usage per query with
set skip_download_if_exceeds_query_cacheto decide what method to take for subsequent reads when the query cache overflows. If it is true, it will not continue to download the cache and read directly from the remote fs. If it is false, it will be replaced in the cache downloaded before the query according to the LRU strategy.Implement
Each query, when executed, requests a temporary context that holds a cached access record (LRU queue) for that query. When the query does not reach the maximum cache usage, it replaces the cache from the main cache queue in the normal way (cold data is selected from all cache blocks according to the LRU algorithm). However, when the query reaches maximum cache usage, it replaces cache blocks downloaded by the query.