Mysql indexes: Difference between revisions
From wikinotes
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= Index Types = | = Index Types = | ||
<blockquote> | <blockquote> | ||
Show the current index using | |||
<syntaxhighlight lang="mysql"> | |||
SHOW INDEX FROM foo_table; | |||
</syntaxhighlight> | |||
== BTree == | == BTree == | ||
<blockquote> | <blockquote> | ||
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But not queries that | But not queries that | ||
<syntaxhighlight lang="yaml"> | <syntaxhighlight lang="yaml"> | ||
- match on only a single key from a multi-key index | - match on only a single key from a multi-key index | ||
(since both keys are hashed together) | (since both keys are hashed together) | ||
- are range bound. (ex. less than 10, betweeen C and F) | - are range bound. (ex. less than 10, betweeen C and F) | ||
(hash-tables are un-ordered, so range queries are not optimized) | (hash-tables are un-ordered, so range queries are not optimized) | ||
</syntaxhighlight> | </syntaxhighlight> | ||
</blockquote><!-- Hash --> | </blockquote><!-- Hash --> | ||
</blockquote><!-- Index Types --> | </blockquote><!-- Index Types --> | ||
Revision as of 20:38, 4 September 2022
Indexes use various datastructures to store data to prevent full scans of all rows in the table.
Index Types
Show the current index using
SHOW INDEX FROM foo_table;BTree
A binary tree. Default for persisted storage engines.
Optimize Queries that are
- sorted - match on field prefixes (but not suffixes) - range bound (less than 10, between C and F)But not queries
- match on field suffixesHash
A hash table. Default for the memory engine.
hash-collisions are accounted for, but each key with the same hash will need to be checked, making the query more expensive.
Indexes have a small memory footprint.Optimize queries that are
- based on the full value (ex. 'IN, NOT IN, =')But not queries that
- match on only a single key from a multi-key index (since both keys are hashed together) - are range bound. (ex. less than 10, betweeen C and F) (hash-tables are un-ordered, so range queries are not optimized)
