Indexes are used to find rows with specific column values quickly. Without an index, MySQL must begin with the first row and then read through the entire table to find the relevant rows. The larger the table, the more this costs. If the table has an index for the columns in question, MySQL can quickly determine the position to seek to in the middle of the data file without having to look at all the data. If a table has 1,000 rows, this is at least 100 times faster than reading sequentially.
In general, indexes are used as described in the following
discussion. Characteristics specific to hash indexes (as used in
MEMORY tables) are described at the end of
MySQL uses indexes for these operations:
To find the rows matching a
To eliminate rows from consideration. If there is a choice between multiple indexes, MySQL normally uses the index that finds the smallest number of rows (the most selective index).
To retrieve rows from other tables when performing joins.
MySQL can use indexes on columns more efficiently if they
are declared as the same type and size. In this context,
CHAR are considered the same
if they are declared as the same size. For example,
CHAR(10) are the same size, but
CHAR(15) are not.
Comparison of dissimilar columns may prevent use of indexes
if values cannot be compared directly without conversion.
Suppose that a numeric column is compared to a string
column. For a given value such as
the numeric column, it might compare equal to any number of
values in the string column such as
'01.e1'. This rules out use of any
indexes for the string column.
To find the
MAX() value for a specific
key_col. This is
optimized by a preprocessor that checks whether you are
WHERE on all key
parts that occur before
in the index. In this case, MySQL does a single key lookup
MAX() expression and replaces
it with a constant. If all expressions are replaced with
constants, the query returns at once. For example:
To sort or group a table if the sorting or grouping is done
on a leftmost prefix of a usable key (for example,
ORDER BY ). If all key
parts are followed by
DESC, the key is
read in reverse order. See
Section 8.13.13, “
ORDER BY Optimization”, and
Section 8.13.14, “
GROUP BY Optimization”.
In some cases, a query can be optimized to retrieve values without consulting the data rows. (An index that provides all the necessary results for a query is called a covering index.) If a query uses only columns from a table that are numeric and that form a leftmost prefix for some key, the selected values can be retrieved from the index tree for greater speed:
Indexes are less important for queries on small tables, or big tables where report queries process most or all of the rows. When a query needs to access most of the rows, reading sequentially is faster than working through an index. Sequential reads minimize disk seeks, even if not all the rows are needed for the query. See Section 126.96.36.199, “How to Avoid Full Table Scans” for details.