Partitioned Tables
A table can be partitioned to increase performance and availability. Partitioning distributes the rows of a table among several sub-tables (partitions). A partitioning scheme determines which rows are sent to which partitions.
After the partitioning scheme is defined, partitioning is managed automatically.
To define a table with partitions, use the PARTITION= option in the CREATE TABLE WITH clause.
When creating tables, NOPARTITION is the default.
Partitioning Schemes
Each dimension of a partitioning scheme defines a rule, or distribution type, for assigning rows to partitions. Conceptually, a dimension defines a set of logical partitions; each logical partition can then be subdivided according to the next dimension's rule. Dimensions are evaluated from left to right.
Four distribution types are available: AUTOMATIC, HASH, LIST, and RANGE. Hash, list, and range are data-dependent and require the ON clause. Automatic distribution is not data dependent and does not allow the ON clause.
An automatic distribution is used when the only goal is to spread rows evenly among the partitions. Rows are arbitrarily assigned to random partitions.
A hash distribution is used to spread rows evenly among the partitions by use of a hash value (instead of randomly). Given a value for the partitioning columns, a query can predict which partition contains the rows that have the matching value. Thus, a query can restrict its search to a subset of partitions.
A list distribution is used to assign rows to partitions based on specific values in one or more columns. A row's partitioning column values are compared to each partition's list values, and when a match is found, the row is sent to that partition. Multiple list values per partition are allowed. If a row matches any of the list values, that partition is selected. One of the partitions must contain the default value in its list; this partition is selected if the row matches none of the list values.
A range distribution is used to assign ranges of values to partitions. The range containing a row's partitioning column values determines the partition in which the row is placed. The ranges must be defined in such a way that every possible value falls into exactly one range. Overlapping ranges are not allowed. Separate ranges cannot map to the same partition—that is, one range, one partition.
A partition defined with values < rangevalue contains all possible values less than rangevalue, down to a smaller rangevalue in the scheme. Similarly, a partition defined with values > rangevalue contains all possible values greater than rangevalue, up to a larger rangevalue in the scheme. Because all values must be covered by a range, the smallest rangevalue must have the operator < (or <=), and the largest rangevalue must have the operator > (or >=). The partitions need not be defined in order of rangevalue.
Multi-column values are tested from left to right. For example, a three-column value (1, 10, 5) is greater than (1, 2, 3000).
While a null can be incorporated into a rangevalue, it is not recommended. The ordering of null relative to non-null values is defined by the SQL standard, so the resulting partitioning is dependent on server implementation.
The optional logical partition names must be unique for each table. The same partition name can occur in other partitioned tables. If a partition name is omitted, the system generates a name (of the form iipartnn).
If NO LOCATION= is specified for a partition, the location list is defined by the enclosing statement's with_clause (that is, the with_clause that contains the PARTITION= clause).
Note: If tables are defined with thousands of partitions, Ingres should be properly configured. Ensure that dmf_tcb_limit is large enough for the expected number of active tables+indexes+partitions. Also ensure that the DMF and RDF cache and memory settings are large enough. Having many partitions makes similar demands on the DBMS Server as does having many tables. The dmf_tcb_limit parameter extends to all TCBs, including partition TCBs. The config.dat default dmf_tcb_limit is 10,000.
Partitioning Syntax
A table partition definition has the following format:
PARTITION = (dimension) | ((dimension) {SUBPARTITION (dimension)})
The syntax for each partition dimension is:
dimension = rule [ON column {, column }]
partitionspec {, partitionspec}
| rule partitionspec {, partitionspec}
rule
Defines the type of distribution scheme for assigning rows to partitions. Valid values are:
AUTOMATIC
Assigns rows arbitrarily to random partitions.
If rule is AUTOMATIC or HASH, the syntax for partitionspec optionally defines the number of partitions and their names:
partitionspec = DEFAULT PARTITIONS | [nn] PARTITION[S] [ ( name {, name} ) ]
[with_clause]
For details, see the description for HASH below.
HASH
Distributes rows evenly among the partitions according to a hash value.
If rule is AUTOMATIC or HASH, the syntax for partitionspec optionally defines the number of partitions and their names:
partitionspec = DEFAULT PARTITIONS | [nn] PARTITION[S] [ ( name {, name} ) ]
[with_clause]
where:
DEFAULT PARTITIONS
Uses the number of partitions specified in the DBMS configuration parameter default_npartitions. The statement returns an error if the default partition value is not set. For more information, see
Default Partition Count. If DEFAULT PARTITIONS is specified, neither explicit partition names nor a with_clause can be specified.
nn
Is the number of partitions, which defaults to 1 if omitted.
name
Identifies the partition and defaults to iipartNN. When the number of partitions is two or more, a comma separated list of names can be provided to override the default value.
LIST
Assigns rows to partitions based on specific values in one or more columns.
If rule is LIST, the syntax for partitionspec defines the list values to be mapped to each partition:
partitionspec = PARTITION [name] VALUES ( listvalue {, listvalue} )
[with_clause]
listvalue = single-constant-value
|
( single-constant-value {, single-constant-value } )
|
DEFAULT
The first form for rangevalue is used to specify only one partitioning column, while the second form is used to specify multiple partitioning columns.
RANGE
Assigns a range of values to each partition.
If rule is RANGE, the syntax for partitionspec defines the ranges that map to each partition:
partitionspec = PARTITION [name] VALUES testing-op rangevalue
[with_clause]
where:
testing-op is one of the operators: < , <= , > , or >=
rangevalue = single-constant-value | (single-constant-value {, single-constant-value } )
The first form for rangevalue is used when there is only one partitioning column, while the second form is used when there are multiple partitioning columns.
with_clause
Specifies WITH clause options for partitioning.
The with_clause for partitioning has the following format:
WITH with-option
where
with-option = LOCATION = ( location { , location } )
Default Partition Count
You can assign a default number of partitions to be used when creating partitioned tables. Set the default value on the DBMS configuration parameter default_npartitions, and then specify WITH PARTITION = (HASH ON column DEFAULT PARTITIONS) when creating or modifying tables. The table will be partitioned into the configured number of partitions. The default_npartitions parameter can be set to an integer between 0 and 32767, inclusive.