mirror of
https://github.com/ClickHouse/ClickHouse.git
synced 2024-12-16 11:22:12 +00:00
131 lines
8.7 KiB
Markdown
131 lines
8.7 KiB
Markdown
---
|
|
slug: /en/operations/query-cache
|
|
sidebar_position: 65
|
|
sidebar_label: Query Cache [experimental]
|
|
---
|
|
|
|
# Query Cache [experimental]
|
|
|
|
The query cache allows to compute `SELECT` queries just once and to serve further executions of the same query directly from the cache.
|
|
Depending on the type of the queries, this can dramatically reduce latency and resource consumption of the ClickHouse server.
|
|
|
|
## Background, Design and Limitations
|
|
|
|
Query caches can generally be viewed as transactionally consistent or inconsistent.
|
|
|
|
- In transactionally consistent caches, the database invalidates (discards) cached query results if the result of the `SELECT` query changes
|
|
or potentially changes. In ClickHouse, operations which change the data include inserts/updates/deletes in/of/from tables or collapsing
|
|
merges. Transactionally consistent caching is especially suitable for OLTP databases, for example
|
|
[MySQL](https://dev.mysql.com/doc/refman/5.6/en/query-cache.html) (which removed query cache after v8.0) and
|
|
[Oracle](https://docs.oracle.com/database/121/TGDBA/tune_result_cache.htm).
|
|
- In transactionally inconsistent caches, slight inaccuracies in query results are accepted under the assumption that all cache entries are
|
|
assigned a validity period after which they expire (e.g. 1 minute) and that the underlying data changes only little during this period.
|
|
This approach is overall more suitable for OLAP databases. As an example where transactionally inconsistent caching is sufficient,
|
|
consider an hourly sales report in a reporting tool which is simultaneously accessed by multiple users. Sales data changes typically
|
|
slowly enough that the database only needs to compute the report once (represented by the first `SELECT` query). Further queries can be
|
|
served directly from the query cache. In this example, a reasonable validity period could be 30 min.
|
|
|
|
Transactionally inconsistent caching is traditionally provided by client tools or proxy packages interacting with the database. As a result,
|
|
the same caching logic and configuration is often duplicated. With ClickHouse's query cache, the caching logic moves to the server side.
|
|
This reduces maintenance effort and avoids redundancy.
|
|
|
|
:::note
|
|
The query cache is an experimental feature that should not be used in production. There are known cases (e.g. in distributed query
|
|
processing) where wrong results are returned.
|
|
:::
|
|
|
|
## Configuration Settings and Usage
|
|
|
|
As long as the result cache is experimental it must be activated using the following configuration setting:
|
|
|
|
```sql
|
|
SET allow_experimental_query_cache = true;
|
|
```
|
|
|
|
Afterwards, setting [use_query_cache](settings/settings.md#use-query-cache) can be used to control whether a specific query or all queries
|
|
of the current session should utilize the query cache. For example, the first execution of query
|
|
|
|
```sql
|
|
SELECT some_expensive_calculation(column_1, column_2)
|
|
FROM table
|
|
SETTINGS use_query_cache = true;
|
|
```
|
|
|
|
will store the query result in the query cache. Subsequent executions of the same query (also with parameter `use_query_cache = true`) will
|
|
read the computed result from the cache and return it immediately.
|
|
|
|
The way the cache is utilized can be configured in more detail using settings [enable_writes_to_query_cache](settings/settings.md#enable-writes-to-query-cache)
|
|
and [enable_reads_from_query_cache](settings/settings.md#enable-reads-from-query-cache) (both `true` by default). The former setting
|
|
controls whether query results are stored in the cache, whereas the latter setting determines if the database should try to 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 not store its
|
|
result in it:
|
|
|
|
```sql
|
|
SELECT some_expensive_calculation(column_1, column_2)
|
|
FROM table
|
|
SETTINGS use_query_cache = true, enable_writes_to_query_cache = false;
|
|
```
|
|
|
|
For maximum control, it is generally recommended to provide settings "use_query_cache", "enable_writes_to_query_cache" and
|
|
"enable_reads_from_query_cache" only with specific queries. It is also possible to enable caching at user or profile level (e.g. via `SET
|
|
use_query_cache = true`) but one should keep in mind that all `SELECT` queries including monitoring or debugging queries to system tables
|
|
may return cached results then.
|
|
|
|
The query cache can be cleared using statement `SYSTEM DROP QUERY CACHE`. The content of the query cache is displayed in system table
|
|
`system.query_cache`. The number of query cache hits and misses are shown as events "QueryCacheHits" and "QueryCacheMisses" in system table
|
|
`system.events`. Both counters are only updated for `SELECT` queries which run with setting "use_query_cache = true". Other queries do not
|
|
affect the cache miss counter.
|
|
|
|
The query cache exists once per ClickHouse server process. However, cache results are by default not shared between users. This can be
|
|
changed (see below) but doing so is not recommended for security reasons.
|
|
|
|
Query results are referenced in the query cache by the [Abstract Syntax Tree (AST)](https://en.wikipedia.org/wiki/Abstract_syntax_tree) 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. To
|
|
make the matching more natural, all query-level settings related to the query cache are removed from the AST.
|
|
|
|
If the query was aborted due to an exception or user cancellation, no entry is written into the query cache.
|
|
|
|
The size of the query cache in bytes, the maximum number of cache entries and the maximum size of individual cache entries (in bytes and in
|
|
records) can be configured using different [server configuration options](server-configuration-parameters/settings.md#server_configuration_parameters_query-cache).
|
|
|
|
To define how long a query must run at least such that its result can be cached, you can use setting
|
|
[query_cache_min_query_duration](settings/settings.md#query-cache-min-query-duration). For example, the result of query
|
|
|
|
``` sql
|
|
SELECT some_expensive_calculation(column_1, column_2)
|
|
FROM table
|
|
SETTINGS use_query_cache = true, query_cache_min_query_duration = 5000;
|
|
```
|
|
|
|
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
|
|
cached - for that use setting [query_cache_min_query_runs](settings/settings.md#query-cache-min-query-runs).
|
|
|
|
Entries in the query cache become stale after a certain time period (time-to-live). By default, this period is 60 seconds but a different
|
|
value can be specified at session, profile or query level using setting [query_cache_ttl](settings/settings.md#query-cache-ttl).
|
|
|
|
Entries in the query cache are 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](settings/settings.md#query-cache-compress-entries).
|
|
|
|
ClickHouse reads table data in blocks of [max_block_size](settings/settings.md#settings-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](settings/settings.md#query-cache-squash-partial-results) (enabled by default) 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 compression rate of cache entries and provides more natural
|
|
block granularity when query results are later served from the query cache.
|
|
|
|
As a result, the query cache stores for each query multiple (partial)
|
|
result blocks. While this behavior is a good default, it can be suppressed using setting
|
|
[query_cache_squash_partial_query_results](settings/settings.md#query-cache-squash-partial-query-results).
|
|
|
|
Also, results of queries with non-deterministic functions such as `rand()` and `now()` are not cached. This can be overruled using
|
|
setting [query_cache_store_results_of_queries_with_nondeterministic_functions](settings/settings.md#query-cache-store-results-of-queries-with-nondeterministic-functions).
|
|
|
|
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
|
|
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
|
|
be marked accessible by other users (i.e. shared) by supplying setting
|
|
[query_cache_share_between_users](settings/settings.md#query-cache-share-between-users).
|
|
|
|
## Related Content
|
|
|
|
- Blog: [Introducing the ClickHouse Query Cache](https://clickhouse.com/blog/introduction-to-the-clickhouse-query-cache-and-design)
|