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How *MySQL* uses indexes
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Indexes are used to find rows with a specific value of one column fast.
Without an index *MySQL* has to start with the first record and then
read through the whole table until it finds the relevent rows. The
bigger the table, the more this costs. If the table has an index for
the colums in question, *MySQL* can get fast a position to seek to in
the middle of the data file without having to look at all the data. If
a table has 1000 rows this is at least 100 times faster than reading
sequentially. Note that if you need to access almost all 1000 rows it
is faster to read sequentially because we then avoid disk seeks.
All *MySQL* indexes (`PRIMARY\', `UNIQUE\' and `INDEX\') are stored in
B-trees. Strings are automatically prefix- and end-space compressed.
*Note `CREATE INDEX\': CREATE INDEX.
Indexes are used to:
* Quickly find the rows that match a `WHERE\' clause.
* Retrieve rows from other tables when performing joins.
* Find the `MAX()\' or `MIN()\' value for a specific indexed column.
SELECT MIN(key_part2),MAX(key_part2) FROM table_name where key_part1=10
* Sort or group a table if the sorting or grouping is done on a
leftmost prefix of a usable key (e.g., `ORDER BY
key_part_1,key_part_2 \'). The key is read in reverse order if all
key parts are followed by `DESC\'.
The index can also be used even if the `ORDER BY\' doesn\'t match
gthe index exactly, as long as all the not used index parts and
all the extra are `ORDER BY\' columns are constants in the `WHERE\'
clause. The following queries will use the index to resolve the
`ORDER BY\' part.
SELECT * FROM foo ORDER BY key_part1,key_part2,key_part3;
SELECT * FROM foo WHERE column=constant ORDER BY column, key_part1;
SELECT * FROM foo WHERE key_part1=const GROUP BY key_part2;
* In some cases a query can be optimized to retrieve values without
consulting the data file. If all used columns for some table are
numeric and form a leftmost prefix for some key, the values may be
retrieved from the index tree for greater speed.
SELECT key_part3 FROM table_name WHERE key_part1=1
Suppose you issue the following `SELECT\' statement:
mysql> SELECT * FROM tbl_name WHERE col1=val1 AND col2=val2;
If a multiple-column index exists on `col1\' and `col2\', the appropriate
rows can be fetched directly. If separate single-column indexes exist
on `col1\' and `col2\', the optimizer tries to find the most restrictive
index by deciding which index will find fewer rows and using that index
to fetch the rows.
If the table has a multiple-column index, any leftmost prefix of the
index can be used by the optimizer to find rows. For example, if you
have a three-column index on `(col1,col2,col3)\', you have indexed
search capabilities on `(col1)\', `(col1,col2)\' and `(col1,col2,col3)\'.
*MySQL* can\'t use a partial index if the columns don\'t form a leftmost
prefix of the index. Suppose you have the `SELECT\' statements shown
below:
mysql> SELECT * FROM tbl_name WHERE col1=val1;
mysql> SELECT * FROM tbl_name WHERE col2=val2;
mysql> SELECT * FROM tbl_name WHERE col2=val2 AND col3=val3;
If an index exists on `(col1,col2,col3)\', only the first query shown
above uses the index. The second and third queries do involve indexed
columns, but `(col2)\' and `(col2,col3)\' are not leftmost prefixes of
`(col1,col2,col3)\'.
*MySQL* also uses indexes for `LIKE\' comparisons if the argument to
`LIKE\' is a constant string that doesn\'t start with a wildcard
character. For example, the following `SELECT\' statements use indexes:
mysql> select * from tbl_name where key_col LIKE "Patrick%";
mysql> select * from tbl_name where key_col LIKE "Pat%_ck%";
In the first statement, only rows with `"Patrick" <= key_col <
"Patricl"\' are considered. In the second statement, only rows with
`"Pat" <= key_col < "Pau"\' are considered.
The following `SELECT\' statements will not use indexes:
mysql> select * from tbl_name where key_col LIKE "%Patrick%";
mysql> select * from tbl_name where key_col LIKE other_col;
In the first statement, the `LIKE\' value begins with a wildcard
character. In the second statement, the `LIKE\' value is not a constant.
Searching using `column_name IS NULL\' will use indexes if column_name
is an index.
*MySQL* normally uses the index that finds least number of rows. An
index is used for columns that you compare with the following operators:
`=\', `>\', `>=\', `<\', `<=\', `BETWEEN\' and a `LIKE\' with a non-wildcard
prefix like `\'something%\'\'.
Any index that doesn\'t span all `AND\' levels in the `WHERE\' clause is
not used to optimize the query. In other words: To be able to use an
index, a prefix of the index must be used in every `AND\' group.
The following `WHERE\' clauses use indexes:
... WHERE index_part1=1 AND index_part2=2 AND other_column=3
... WHERE index=1 OR A=10 AND index=2 /* index = 1 OR index = 2 */
... WHERE index_part1=\'hello\' AND index_part_3=5
/* optimized like "index_part1=\'hello\'" */
... WHERE index1=1 and index2=2 or index1=3 and index3=3;
/* Can use index on index1 but not on index2 or index 3 */
These `WHERE\' clauses do *NOT* use indexes:
... WHERE index_part2=1 AND index_part3=2 /* index_part_1 is not used */
... WHERE index=1 OR A=10 /* Index is not used in both AND parts */
... WHERE index_part1=1 OR index_part2=10 /* No index spans all rows */
Note that in some cases `MySQL\' will not use an index, even if one
would be available. Some of the cases where this happens are:
* If the use of the index, would require *MySQL* to access more than
30 % of the rows in the table. (In this case a table scan is
probably much faster as this will require us to do much fewer
seeks). Note that if you with such a query use `LIMIT\' to only
retrieve part of the rows, *MySQL* will use an index anyway as it
can this way much more quickly find the few rows to return in the
result.
