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
IN, and so on, in
The maximum number of indexes per table and the maximum index length is defined per storage engine. See 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.
syntax in an index specification, you can create an index that
uses only the first
N characters of a
string column. Indexing only a prefix of column values in this
way can make the index file much smaller. When you index a
TEXT column, you
must specify a prefix length for the index.
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
Prefix limits are measured in bytes, while the prefix length
CREATE TABLE statements is
interpreted as number of characters. Take this into
account when specifying a prefix length for a column that uses
a multibyte character set.
You can also create
FULLTEXT indexes. These
are used for full-text searches. Only the
MyISAM storage engines support
FULLTEXT indexes and only for
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”.
Optimizations are applied to certain kinds of
FULLTEXT queries against single
InnoDB tables. Queries with these
characteristics are particularly efficient:
FULLTEXT queries that only return the
document ID, or the document ID and the search rank.
FULLTEXT queries that sort the matching
rows in descending order of score and apply a
LIMIT clause to take the top N matching
rows. For this optimization to apply, there must be no
WHERE clauses and only a single
ORDER BY clause in descending order.
FULLTEXT queries that retrieve only the
COUNT(*) value of rows matching a search
term, with no additional
WHERE clause as
> 0 comparison operator.
You can also create indexes on spatial data types. Currently,
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).
MEMORY storage engine uses
HASH indexes by default, but also supports
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