Mysql caches: Difference between revisions

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= Query Cache (<8.0) =
There are some general MySQL caches, but increasingly they are engine specific.
<blockquote>
{{ NOTE |
It is disabled by default starting in MySQL-5.6<br>
https://dev.mysql.com/blog-archive/mysql-8-0-retiring-support-for-the-query-cache/
}}


The query cache is a key-value store of cacheable-queries, and their result.<br>
{{ TODO |
This interferes with benchmarking, since repeat queries may be much faster to lookup.
split up this page per database engine, it's hard to read }}


To avoid the query cache:
= Documentation =
<syntaxhighlight lang="mysql">
<blockquote>
# 1. Add a calculated-function to your selected rows - since they are not cacheable
{| class="wikitable"
#    TODO: validate
|-
SELECT ..., NOW() FROM ...
| MySQL: Query Cache || https://dev.mysql.com/doc/refman/5.7/en/query-cache.html
|-
| MyISAM: Key Cache || https://dev.mysql.com/doc/refman/5.7/en/myisam-key-cache.html


# 2. Use 'SQL_NO_CACHE' in your query
|}
#    (This often has not worked for me)
</blockquote><!-- Documentation -->
SELECT SQL_NO_CACHE ... FROM ...
</syntaxhighlight>


It is implemented as an LRU key-value store for exact (byte-for-byte) query matches.<br>
= General =
Each cache entry knows which tables it references for permissions, and cache-invalidation.<br>
<blockquote>
Whenever any row is changed in a table, all cache entries using that table are purged.
{| class="wikitable"
|-
| [[mysql query cache]]
|-
| [[mysql os cache]]
|-
|}


<syntaxhighlight lang="mysql">
</blockquote><!-- General -->
SHOW VARIABLES LIKE 'query_cache_type';  # cache is 'ON/OFF/DEMAND'
SHOW VARIABLES LIKE 'query_cache_size';  # max capacity of cache
</syntaxhighlight>
</blockquote><!-- The Query Cache -->


= The OS Cache =
= Engine Specific =
<blockquote>
<blockquote>
{{ TODO |
{| class="wikitable"
research }}
|-
</blockquote><!-- The OS Cache -->
| [[mysql innodb caches]]
|-
| [[mysql myisam caches]]
|-
|}
 
</blockquote><!-- Engine Specific -->

Latest revision as of 04:25, 8 September 2022

There are some general MySQL caches, but increasingly they are engine specific.

TODO:

split up this page per database engine, it's hard to read

Documentation

MySQL: Query Cache https://dev.mysql.com/doc/refman/5.7/en/query-cache.html
MyISAM: Key Cache https://dev.mysql.com/doc/refman/5.7/en/myisam-key-cache.html

General

mysql query cache
mysql os cache

Engine Specific

mysql innodb caches
mysql myisam caches
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