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Merge branch 'master' into add-support-for-settings-alias
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Antonio Andelic
commit
90e31a023e
@ -48,6 +48,7 @@ RUN apt-get update \
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gdb \
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git \
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gperf \
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libclang-rt-${LLVM_VERSION}-dev \
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lld-${LLVM_VERSION} \
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llvm-${LLVM_VERSION} \
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llvm-${LLVM_VERSION}-dev \
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@ -22,6 +22,6 @@ Additional cache types:
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- [Dictionaries](../sql-reference/dictionaries/index.md) data cache.
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- Schema inference cache.
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- [Filesystem cache](storing-data.md) over S3, Azure, Local and other disks.
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- [(Experimental) Query result cache](query-result-cache.md).
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- [(Experimental) Query cache](query-cache.md).
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To drop one of the caches, use [SYSTEM DROP ... CACHE](../sql-reference/statements/system.md#drop-mark-cache) statements.
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@ -1,112 +0,0 @@
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---
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slug: /en/operations/query-result-cache
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sidebar_position: 65
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sidebar_label: Query Result Cache [experimental]
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---
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# Query Result Cache [experimental]
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The query result cache allows to compute `SELECT` queries just once and to serve further executions of the same query directly from the
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cache. Depending on the type of the queries, this can dramatically reduce latency and resource consumption of the ClickHouse server.
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## Background, Design and Limitations
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Query result caches can generally be viewed as transactionally consistent or inconsistent.
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- In transactionally consistent caches, the database invalidates (discards) cached query results if the result of the `SELECT` query changes
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or potentially changes. In ClickHouse, operations which change the data include inserts/updates/deletes in/of/from tables or collapsing
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merges. Transactionally consistent caching is especially suitable for OLTP databases, for example
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[MySQL](https://dev.mysql.com/doc/refman/5.6/en/query-cache.html) (which removed query result cache after v8.0) and
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[Oracle](https://docs.oracle.com/database/121/TGDBA/tune_result_cache.htm).
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- In transactionally inconsistent caches, slight inaccuracies in query results are accepted under the assumption that all cache entries are
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assigned a validity period after which they expire (e.g. 1 minute) and that the underlying data changes only little during this period.
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This approach is overall more suitable for OLAP databases. As an example where transactionally inconsistent caching is sufficient,
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consider an hourly sales report in a reporting tool which is simultaneously accessed by multiple users. Sales data changes typically
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slowly enough that the database only needs to compute the report once (represented by the first `SELECT` query). Further queries can be
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served directly from the query result cache. In this example, a reasonable validity period could be 30 min.
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Transactionally inconsistent caching is traditionally provided by client tools or proxy packages interacting with the database. As a result,
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the same caching logic and configuration is often duplicated. With ClickHouse's query result cache, the caching logic moves to the server
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side. This reduces maintenance effort and avoids redundancy.
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:::warning
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The query result cache is an experimental feature that should not be used in production. There are known cases (e.g. in distributed query
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processing) where wrong results are returned.
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:::
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## Configuration Settings and Usage
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As long as the result cache is experimental it must be activated using the following configuration setting:
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```sql
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SET allow_experimental_query_result_cache = true;
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```
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Afterwards, setting [use_query_result_cache](settings/settings.md#use-query-result-cache) can be used to control whether a specific query or
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all queries of the current session should utilize the query result cache. For example, the first execution of query
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```sql
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SELECT some_expensive_calculation(column_1, column_2)
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FROM table
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SETTINGS use_query_result_cache = true;
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```
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will store the query result in the query result cache. Subsequent executions of the same query (also with parameter `use_query_result_cache
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= true`) will read the computed result from the cache and return it immediately.
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The way the cache is utilized can be configured in more detail using settings [enable_writes_to_query_result_cache](settings/settings.md#enable-writes-to-query-result-cache)
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and [enable_reads_from_query_result_cache](settings/settings.md#enable-reads-from-query-result-cache) (both `true` by default). The first
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settings controls whether query results are stored in the cache, whereas the second parameter determines if the database should try to
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retrieve query results from the cache. For example, the following query will use the cache only passively, i.e. attempt to read from it but
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not store its result in it:
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```sql
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SELECT some_expensive_calculation(column_1, column_2)
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FROM table
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SETTINGS use_query_result_cache = true, enable_writes_to_query_result_cache = false;
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```
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For maximum control, it is generally recommended to provide settings "use_query_result_cache", "enable_writes_to_query_result_cache" and
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"enable_reads_from_query_result_cache" only with specific queries. It is also possible to enable caching at user or profile level (e.g. via
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`SET use_query_result_cache = true`) but one should keep in mind that all `SELECT` queries including monitoring or debugging queries to
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system tables may return cached results then.
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The query result cache can be cleared using statement `SYSTEM DROP QUERY RESULT CACHE`. The content of the query result cache is displayed
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in system table `SYSTEM.QUERY_RESULT_CACHE`. The number of query result cache hits and misses are shown as events "QueryCacheHits" and
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"QueryCacheMisses" in system table `SYSTEM.EVENTS`. Both counters are only updated for `SELECT` queries which run with setting
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"use_query_result_cache = true". Other queries do not affect the cache miss counter.
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The query result cache exists once per ClickHouse server process. However, cache results are by default not shared between users. This can
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be changed (see below) but doing so is not recommended for security reasons.
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Query results are referenced in the query result cache by the [Abstract Syntax Tree (AST)](https://en.wikipedia.org/wiki/Abstract_syntax_tree)
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of their query. This means that caching is agnostic to upper/lowercase, for example `SELECT 1` and `select 1` are treated as the same query.
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To make the matching more natural, all query-level settings related to the query result cache are removed from the AST.
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If the query was aborted due to an exception or user cancellation, no entry is written into the query result cache.
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The size of the query result cache, the maximum number of cache entries and the maximum size of cache entries (in bytes and in records) can
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be configured using different [server configuration options](server-configuration-parameters/settings.md#server_configuration_parameters_query-result-cache).
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To define how long a query must run at least such that its result can be cached, you can use setting
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[query_result_cache_min_query_duration](settings/settings.md#query-result-cache-min-query-duration). For example, the result of query
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``` sql
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SELECT some_expensive_calculation(column_1, column_2)
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FROM table
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SETTINGS use_query_result_cache = true, query_result_cache_min_query_duration = 5000;
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```
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is only cached if the query runs longer than 5 seconds. It is also possible to specify how often a query needs to run until its result is
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cached - for that use setting [query_result_cache_min_query_runs](settings/settings.md#query-result-cache-min-query-runs).
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Entries in the query result cache become stale after a certain time period (time-to-live). By default, this period is 60 seconds but a
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different value can be specified at session, profile or query level using setting [query_result_cache_ttl](settings/settings.md#query-result-cache-ttl).
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Also, results of queries with non-deterministic functions such as `rand()` and `now()` are not cached. This can be overruled using
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setting [query_result_cache_store_results_of_queries_with_nondeterministic_functions](settings/settings.md#query-result-cache-store-results-of-queries-with-nondeterministic-functions).
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Finally, entries in the query cache are not shared between users due to security reasons. For example, user A must not be able to bypass a
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row policy on a table by running the same query as another user B for whom no such policy exists. However, if necessary, cache entries can
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be marked accessible by other users (i.e. shared) by supplying setting
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[query_result_cache_share_between_users](settings/settings.md#query-result-cache-share-between-users).
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@ -1303,7 +1303,7 @@ Default value: `3`.
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## use_query_cache {#use-query-cache}
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If turned on, `SELECT` queries may utilize the [query cache](../query-cache.md). Parameters [enable_reads_from_query_cache](#enable-readsfrom-query-cache)
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If turned on, `SELECT` queries may utilize the [query cache](../query-cache.md). Parameters [enable_reads_from_query_cache](#enable-reads-from-query-cache)
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and [enable_writes_to_query_cache](#enable-writes-to-query-cache) control in more detail how the cache is used.
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Possible values:
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