17.2.3.1. LINEAR HASH Partitioning
MySQL also supports linear hashing, which differs from regular
hashing in that linear hashing utilizes a linear powers-of-two
algorithm whereas regular hashing employs the modulus of the
hashing function's value.
Syntactically, the only difference between linear-hash
partitioning and regular hashing is the addition of the
LINEAR keyword in the PARTITION
BY clause, as shown here:
CREATE TABLE employees (
id INT NOT NULL,
fname VARCHAR(30),
lname VARCHAR(30),
hired DATE NOT NULL DEFAULT '1970-01-01',
separated DATE NOT NULL DEFAULT '9999-12-31',
job_code INT,
store_id INT
)
PARTITION BY LINEAR HASH( YEAR(hired) )
PARTITIONS 4;
Given an expression expr, the
partition in which the record is stored when linear hashing is
used is partition number N from
among num partitions, where
N is derived according to the
following algorithm:
-
Find the next power of 2 greater than
num. We call this value
V; it can be calculated as:
V = POWER(2, CEILING(LOG(2, num)))
(For example, suppose that num
is 13. Then LOG(2,13) is
3.7004397181411.
CEILING(3.7004397181411) is 4, and
V =
POWER(2,4), which is 16.)
Set N =
F(column_list)
& (V - 1).
-
While N >=
num:
Set V =
CEIL(V / 2)
Set N =
N &
(V - 1)
For example, suppose that the table t1,
using linear hash partitioning and having 6 partitions, is
created using this statement:
CREATE TABLE t1 (col1 INT, col2 CHAR(5), col3 DATE)
PARTITION BY LINEAR HASH( YEAR(col3) )
PARTITIONS 6;
Now assume that you want to insert two records into
t1 having the col3
column values '2003-04-14' and
'1998-10-19'. The partition number for the
first of these is determined as follows:
V = POWER(2, CEILING( LOG(2,7) )) = 8
N = YEAR('2003-04-14') & (8 - 1)
= 2003 & 7
= 3
(3 >= 6 is FALSE: record stored in partition #3)
The number of the partition where the second record is stored
is calculated as shown here:
V = 8
N = YEAR('1998-10-19') & (8-1)
= 1998 & 7
= 6
(6 >= 6 is TRUE: additional step required)
N = 6 & CEILING(5 / 2)
= 6 & 3
= 2
(2 >= 6 is FALSE: record stored in partition #2)
The advantage in partitioning by linear hash is that the
adding, dropping, merging, and splitting of partitions is made
much faster, which can be beneficial when dealing with tables
containing extremely large amounts (terabytes) of data. The
disadvantage is that data is less likely to be evenly
distributed between partitions as compared with the
distribution obtained using regular hash partitioning.
