The most common type of index involves a single column, storing
copies of the values from that column in a data structure,
allowing fast lookups for the rows with the corresponding column
values. The B-tree data structure lets the index quickly find a
specific value, a set of values, or a range of values,
corresponding to operators such as =,
>, ≤,
BETWEEN, IN, and so on, in
a WHERE clause.
The maximum number of indexes per table and the maximum index length is defined per storage engine. See Chapter 14, The InnoDB Storage Engine, and Chapter 15, Alternative Storage Engines. All storage engines support at least 16 indexes per table and a total index length of at least 256 bytes. Most storage engines have higher limits.
For additional information about column indexes, see Section 13.1.13, “CREATE INDEX Syntax”.
With
col_name(N)N characters of the column. Indexing
only a prefix of column values in this way can make the index
file much smaller. When you index a
BLOB or
TEXT column, you
must specify a prefix length for the index.
For example:
CREATE TABLE test (blob_col BLOB, INDEX(blob_col(10)));
Prefixes can be up to 1000 bytes long (767 bytes for
InnoDB tables, unless you have
innodb_large_prefix set).
Prefix limits are measured in bytes, whereas the prefix length
in CREATE TABLE,
ALTER TABLE, and
CREATE INDEX statements is
interpreted as number of characters for nonbinary string types
(CHAR,
VARCHAR,
TEXT) and number of bytes for
binary string types (BINARY,
VARBINARY,
BLOB). Take this into account
when specifying a prefix length for a nonbinary string column
that uses a multibyte character set.
For additional information about index prefixes, see Section 13.1.13, “CREATE INDEX Syntax”.
FULLTEXT indexes are used for full-text
searches. Only the MyISAM storage engine
supports FULLTEXT indexes and only for
CHAR,
VARCHAR, and
TEXT columns. Indexing always
takes place over the entire column and column prefix indexing is
not supported. For details, see
Section 12.9, “Full-Text Search Functions”.
You can create indexes on spatial data types. Only
MyISAM supports R-tree indexes on spatial
types. Other storage engines use B-trees for indexing spatial
types (except for ARCHIVE, which does not
support spatial type indexing).
The MEMORY storage engine uses
HASH indexes by default, but also supports
BTREE indexes.