CREATE TABLE PARTITION

Function

CREATE TABLE PARTITION creates a partitioned table. Partitioning refers to splitting what is logically one large table into smaller physical pieces based on specific schemes. The table based on the logic is called a partition cable, and a physical piece is called a partition. Data is stored on these smaller physical pieces, namely, partitions, instead of the larger logical partitioned table.

The common forms of partitioning include range partitioning, hash partitioning, list partitioning, and value partitioning. Currently, row-store and column-store tables support only range partitioning.

In range partitioning, the table is partitioned into ranges defined by a key column or set of columns, with no overlap between the ranges of values assigned to different partitions. Each range has a dedicated partition for data storage.

The partitioning policy for Range Partitioning refers to how data is inserted into partitions. Currently, range partitioning only allows the use of the range partitioning policy.

Range partitioning policy: Data is mapped to a created partition based on the partition key value. If the data can be mapped to, it is inserted into the specific partition; if it cannot be mapped to, error messages are returned. This is the most commonly used partitioning policy.

Partitioning can provide several benefits:

• Query performance can be improved dramatically in certain situations, particularly when most of the heavily accessed rows of the table are in a single partition or a small number of partitions. Partitioning narrows the range of data search and improves data access efficiency.
• When queries or updates access a large percentage of a single partition, performance can be improved by taking advantage of sequential scan of that partition instead of reads scattered across the whole table.
• Bulk loads and deletion can be performed by adding or removing partitions, if that requirement is planned into the partitioning design. It also entirely avoids the VACUUM overhead caused by a bulk DELETE (only for range partitioning).

Precautions

A partitioned table supports unique and primary key constraints. The constraint keys of these constraints contain all partition keys.

Syntax

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CREATE TABLE [ IF NOT EXISTS ] partition_table_name
( [ 
    { column_name data_type [ COLLATE collation ] [ column_constraint [ ... ] ]
    | table_constraint
    | LIKE source_table [ like_option [...] ] }[, ... ]
] )
    [ WITH ( {storage_parameter = value} [, ... ] ) ]
    [ COMPRESS | NOCOMPRESS ]
    [ TABLESPACE tablespace_name ]
    [ DISTRIBUTE BY { REPLICATION | { [ HASH ] ( column_name ) } } ]
    [ TO { GROUP groupname | NODE ( nodename [, ... ] ) } ]
    PARTITION BY { 
        {VALUES (partition_key)} |
        {RANGE (partition_key) ( partition_less_than_item [, ... ] )} |
        {RANGE (partition_key) ( partition_start_end_item [, ... ] )}
    } [ { ENABLE | DISABLE } ROW MOVEMENT ]; 

• column_constraint is as follows:

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  [ CONSTRAINT constraint_name ] { NOT NULL | NULL | CHECK ( expression ) | DEFAULT default_expr | UNIQUE index_parameters | PRIMARY KEY index_parameters } [ DEFERRABLE | NOT DEFERRABLE | INITIALLY DEFERRED | INITIALLY IMMEDIATE ]

• table_constraint is as follows:

  [ CONSTRAINT constraint_name ]
  { CHECK ( expression ) | 
    UNIQUE ( column_name [, ... ] ) index_parameters | 
    PRIMARY KEY ( column_name [, ... ] ) index_parameters}
  [ DEFERRABLE | NOT DEFERRABLE | INITIALLY DEFERRED | INITIALLY IMMEDIATE ]

• like_option is as follows:

  1	{ INCLUDING | EXCLUDING } { DEFAULTS | CONSTRAINTS | INDEXES | STORAGE | COMMENTS | RELOPTIONS | DISTRIBUTION | ALL }

• index_parameters is as follows:

  1 2	[ WITH ( {storage_parameter = value} [, ... ] ) ] [ USING INDEX TABLESPACE tablespace_name ]

Parameter Description

• IF NOT EXISTS

  Does not throw an error if a table with the same name exists. A notice is issued in this case.

• partition_table_name

  Name of the partitioned table

  Value range: a string. It must comply with the naming convention.

• column_name

  Specifies the name of a column to be created in the new table.

  Value range: a string. It must comply with the naming convention.

• data_type

  Specifies the data type of the column.

• COLLATE collation

  Assigns a collation to the column (which must be of a collatable data type). If no collation is specified, the default collation is used.

  The collatable types are char, varchar, text, nchar, and nvarchar.

• CONSTRAINT constraint_name

  Specifies a name for a column or table constraint. The optional constraint clauses specify constraints that new or updated rows must satisfy for an insert or update operation to succeed.

  There are two ways to define constraints:

  • A column constraint is defined as part of a column definition, and it is bound to a particular column.
  • A table constraint is not bound to any particular columns but can apply to more than one column.
• LIKE source_table [ like_option ... ]

  Specifies a table from which the new table automatically copies all column names, their data types, and their not-null constraints.

  Unlike INHERITS, the new table and original table are decoupled after creation is complete. Changes to the original table will not be applied to the new table, and it is not possible to include data of the new table in scans of the original table.

  Default expressions for the copied column definitions will only be copied if INCLUDING DEFAULTS is specified. The default behavior is to exclude default expressions, resulting in the copied columns in the new table having default values NULL.

  NOT NULL constraints are always copied to the new table. CHECK constraints will only be copied if INCLUDING CONSTRAINTS is specified; other types of constraints will never be copied. These rules also apply to column constraints and table constraints.

  Unlike INHERITS, columns and constraints copied by LIKE are not merged with similarly named columns and constraints. If the same name is specified explicitly or in another LIKE clause, an error is reported.

  • Any indexes on the source table will not be created on the new table, unless the INCLUDING INDEXES clause is specified.
  • STORAGE settings for the copied column definitions will only be copied if INCLUDING STORAGE is specified. The default behavior is to exclude STORAGE settings.
  • Comments for the copied columns, constraints, and indexes will only be copied if INCLUDING COMMENTS is specified. The default behavior is to exclude comments.
  • If INCLUDING RELOPTIONS is specified, the new table will copy the storage parameter (WITH clause of the source table) of the source table. The default behavior is to exclude partition definition of the storage parameter of the source table.
  • If INCLUDING DISTRIBUTION is specified, the new table will copy the distribution information of the source table, including distribution type and column, and the new table cannot use DISTRIBUTE BY clause. The default behavior is to exclude distribution information of the source table.
  • INCLUDING ALL is an abbreviated form of INCLUDING DEFAULTS INCLUDING CONSTRAINTS INCLUDING INDEXES INCLUDING STORAGE INCLUDING COMMENTS INCLUDING RELOPTIONS INCLUDING DISTRIBUTION.
• WITH ( storage_parameter [= value] [, ... ] )

  Specifies an optional storage parameter for a table or an index. Optional parameters are as follows:

  • FILLFACTOR

    The fillfactor of a table is a percentage between 10 and 100. 100 (complete packing) is the default value. When a smaller fillfactor is specified, INSERT operations pack table pages only to the indicated percentage. The remaining space on each page is reserved for updating rows on that page. This gives UPDATE a chance to place the updated copy of a row on the same page, which is more efficient than placing it on a different page. For a table whose records are never updated, setting the fillfactor to 100 (complete packing) is the appropriate choice, but in heavily updated tables smaller fillfactors are appropriate. The parameter has no meaning for column-store tables.

    Value range: 10–100

  • ORIENTATION

    Determines the storage mode of the data in the table.

    Valid value:

    • COLUMN: The data will be stored in columns.
    • ROW (default value): The data will be stored in rows.
    • ORC: The data of the table will be stored in ORC format (only HDFS table).
      

      orientation cannot be modified.

  • COMPRESSION
    • The valid values for column-store tables are YES/NO and LOW/MIDDLE/HIGH, and the default is LOW.
    • The valid values for row-store tables are YES and NO, and the default is NO.
      

      The row-store table compression function is not put into commercial use. To use this function, contact technical support engineers.

  • MAX_BATCHROW

    Specifies the maximum of a storage unit during data loading process. The parameter is only valid for column-store table.

    Value range: 10000 to 60000

    Default value: 60000

  • PARTIAL_CLUSTER_ROWS

    Specifies the number of records to be partial cluster stored during data loading process. The parameter is only valid for column-store table.

    Value range: The valid value is no less than 100000. The value is the multiple of MAX_BATCHROW.

  • enable_delta

    Specifies whether to enable delta tables in column-store tables. The parameter is only valid for column-store tables.

    Default value: off

  • DELTAROW_THRESHOLD

    A reserved parameter. The parameter is only valid for column-store table.

    The value ranges from 0 to 60000. The default value is 6000.

  • COLD_TABLECPACE

    Specifies the OBS tablespace for the cold partitions in a hot or cold table. This parameter is available only to partitioned column-store tables and cannot be modified. It must be used together with storage_policy.

    Valid value: a valid OBS tablespace name

  • STORAGE_POLICY

    Specifies the rule for switching between hot and cold partitions. This parameter is used only for multi-temperature tables. It must be used together with cold_tablespace.

    Value range: Cold and hot switchover policy name:Cold and hot switchover threshold. Currently, only LMT and HPN policies are supported. LMT indicates that the switchover is performed based on the last update time of partitions. HPN indicates the switchover is performed based on a fixed number of reserved hot partitions.

    • LMT:[day]: Switch the hot partition data that is not updated in the last [day] days to the OBS tablespace as cold partition data. [day] is an integer ranging from 0 to 36500, in days.
    • HPN:[hot_partition_num]: [hot_partition_num] indicates the number of hot partitions (with data) to be retained. The rule is to find the maximum sequence ID of the partitions with data. The partitions without data whose sequence ID is greater than the maximum sequence ID are hot partitions, and [hot_partition_num] partitions are retained as hot partitions in descending order according to the sequence ID. A partition whose sequence ID is smaller than the minimum sequence ID of the retained hot partition is a cold partition. During hot and cold partition switchover, data needs to be migrated to the OBS tablespace. [hot_partition_num] is an integer ranging from 0 to 1600.
  • COLVERSION

    Specifies the version of the column-store format. Switching between different storage formats is supported. However, the storage format of a partitioned table cannot be switched.

    Valid value:

    1.0: Each column in a column-store table is stored in a separate file. The file name is relfilenode.C1.0, relfilenode.C2.0, relfilenode.C3.0, or similar.

    2.0: All columns of a column-store table are combined and stored in a file. The file is named relfilenode.C1.0.

    Default value: 2.0

    The value of COLVERSION can only be set to 2.0 for OBS hot and cold tables.

    

    When creating a column-store table, set COLVERSION to 2.0. Compared with the 1.0 storage format, the performance is significantly improved:

    1. The time required for creating a column-store wide table is significantly reduced.
    2. In the Roach data backup scenario, the backup time is significantly reduced.
    3. The build and catch up time is greatly reduced.
    4. The occupied disk space decreases significantly.
  • SKIP_FPI_HINT

    Indicates whether to skip the hint bits operation when the full-page writes (FPW) log needs to be written during sequential scanning.

    Default value: false

    

    If SKIP_FPI_HINT is set to true and the checkpoint operation is performed on a table, no Xlog will be generated when the table is sequentially scanned. This applies to intermediate tables that are queried less frequently, reducing the size of Xlogs and improving query performance.

• COMPRESS / NOCOMPRESS

  Specifies the keyword COMPRESS during the creation of a table, so that the compression feature is triggered in the case of a bulk INSERT operation. If this feature is enabled, a scan is performed for all tuple data within the page to generate a dictionary and then the tuple data is compressed and stored. If NOCOMPRESS is specified, the table is not compressed.

  Default value: NOCOMPRESS, tuple data is not compressed before storage.

• TABLESPACE tablespace_name

  Specifies the new table will be created in tablespace_name tablespace. If not specified, default tablespace is used. The OBS tablespace is not supported.

• DISTRIBUTE BY

  Specifies how the table is distributed or replicated between DNs.

  Valid value:

  • REPLICATION: Each row in the table exists on all DNs, that is, each DN has complete table data.
  • HASH (column_name): Each row of the table will be placed into all the DNs based on the hash value of the specified column.
  

  • When DISTRIBUTE BY HASH (column_name) is specified, the primary key and its unique index must contain the column_name column.
  • When DISTRIBUTE BY HASH (column_name) in a referenced table is specified, the foreign key of the reference table must contain the column_name column.

  Default value: HASH(column_name), the key column of column_name (if any) or the column of distribution column supported by first data type.

  column_name supports the following data types:

  • INTEGER TYPES: TINYINT, SMALLINT, INT, BIGINT, NUMERIC/DECIMAL
  • CHARACTER TYPES: CHAR, BPCHAR, VARCHAR, VARCHAR2, NVARCHAR2
  • DATA/TIME TYPES: DATE, TIME, TIMETZ, TIMESTAMP, TIMESTAMPTZ, INTERVAL, SMALLDATETIME
• TO { GROUP groupname | NODE ( nodename [, ... ] ) }

  TO GROUP specifies the Node Group in which the table is created. Currently, it cannot be used for HDFS tables. TO NODE is used for internal scale-out tools.

• PARTITION BY RANGE(partition_key)

  Creates a range partition. partition_key is the name of the partition key.

  (1) Assume that the VALUES LESS THAN syntax is used.

  

  In this case, a maximum of four partition keys are supported.

  Data types supported by the partition keys are as follows: SMALLINT, INTEGER, BIGINT, DECIMAL, NUMERIC, REAL, DOUBLE PRECISION, CHARACTER VARYING(n), VARCHAR(n), CHARACTER(n), CHAR(n), CHARACTER, CHAR, TEXT, NVARCHAR2, NAME, TIMESTAMP[(p)] [WITHOUT TIME ZONE], TIMESTAMP[(p)] [WITH TIME ZONE], and DATE.

  (2) Assume that the START END syntax is used.

  

  In this case, only one partition key is supported.

  Data types supported by the partition key are as follows: SMALLINT, INTEGER, BIGINT, DECIMAL, NUMERIC, REAL, DOUBLE PRECISION, TIMESTAMP[(p)] [WITHOUT TIME ZONE], TIMESTAMP[(p)] [WITH TIME ZONE], and DATE.

• PARTITION partition_name VALUES LESS THAN ( { partition_value | MAXVALUE } )

  Specifies the information of partitions. partition_name is the name of a range partition. partition_value is the upper limit of range partition, and the value depends on the type of partition_key. MAXVALUE can specify the upper boundary of a range partition, and it is commonly used to specify the upper boundary of the last range partition.

  

  • Upper boundaries must be specified for each partition.
  • The types of upper boundaries must be the same as those of partition keys.
  • In a partition list, partitions are arranged in ascending order of upper boundary values. Therefore, a partition with a certain upper boundary value is placed before another partition with a larger upper boundary value.
  • If a partition key consists of multiple columns, the columns are used for partitioning in sequence. The first column is preferred to be used for partitioning. If the values of the first columns are the same, the second column is used. The subsequent columns are used in the same manner.
• PARTITION partition_name {START (partition_value) END (partition_value) EVERY (interval_value)} | {START (partition_value) END (partition_value|MAXVALUE)} | {START(partition_value)} | {END (partition_value | MAXVALUE)}

  Specifies partition definitions.

  • partition_name: name or name prefix of a range partition. It is the name prefix only in the following cases (assuming that partition_name is p1):
    • If START+END+EVERY is used, the names of partitions will be defined as p1_1, p1_2, and the like. For example, if PARTITION p1 START(1) END(4) EVERY(1) is defined, the generated partitions are [1, 2), [2, 3), and [3, 4), and their names are p1_1, p1_2, and p1_3. In this case, p1 is a name prefix.
    • If the defined statement is in the first place and has START specified, the range (MINVALUE, START) will be automatically used as the first actual partition, and its name will be p1_0. The other partitions are then named p1_1, p1_2, and the like. For example, if PARTITION p1 START(1), PARTITION p2 START(2) is defined, generated partitions are (MINVALUE, 1), [1, 2), and [2, MAXVALUE), and their names will be p1_0, p1_1, and p2. In this case, p1 is a name prefix and p2 is a partition name. MINVALUE means the minimum value.
  • partition_value: start point value or end point value of a range partition. The value depends on partition_key and cannot be MAXVALUE.
  • interval_value: width of each partition for dividing the [START, END) range. It cannot be MAXVALUE. If the value of (END – START) divided by EVERY has a remainder, the width of only the last partition is less than the value of EVERY.
  • MAXVALUE: maximum value. It is usually used to set the upper boundary for the last range partition.
  

  1. If the defined statement is in the first place and has START specified, the range (MINVALUE, START) will be automatically used as the first actual partition.
  2. The START END syntax must comply with the following rules:
     • The value of START (if any, same for the following situations) in each partition_start_end_item must be smaller than that of END.
     • In two adjacent partition_start_end_item statements, the value of the first END must be equal to that of the second START.
     • The value of EVERY in each partition_start_end_item must be a positive number (in ascending order) and must be smaller than END minus START.
     • Each partition includes the start value (unless it is MINVALUE) and excludes the end value. The format is as follows: [Start value, end value).
     • Partitions created by the same partition_start_end_item belong to the same tablespace.
     • If partition_name is a name prefix of a partition, the length must not exceed 57 bytes. If there are more than 57 bytes, the prefix will be automatically truncated.
     • When creating or modifying a partitioned table, ensure that the total number of partitions in the table does not exceed the maximum value (32767).
  3. In statements for creating partitioned tables, START END and LESS THAN cannot be used together.
  4. The START END syntax in a partitioned table creation SQL statement will be replaced with the VALUES LESS THAN syntax when gs_dump is executed.
• { ENABLE | DISABLE } ROW MOVEMENT

  Specifies the row movement switch.

  If the tuple value is updated on the partition key during the UPDATE action, the partition where the tuple is located is altered. Setting of this parameter enables error messages to be reported or movement of the tuple between partitions.

  Valid value:

  • ENABLE: Row movement is enabled.
  • DISABLE (default value): Disable row movement.

Examples

• Example 1: Create a range-partitioned table tpcds.web_returns_p1. The table has eight partitions and the data type of their partition key is integer. The ranges of the partitions are: wr_returned_date_sk < 2450815, 2450815 ≤ wr_returned_date_sk < 2451179, 2451179 ≤ wr_returned_date_sk < 2451544, 2451544 ≤ wr_returned_date_sk < 2451910, 2451910 ≤ wr_returned_date_sk < 2452275, 2452275 ≤ wr_returned_date_sk < 2452640, 2452640 ≤ wr_returned_date_sk < 2453005, and wr_returned_date_sk ≥ 2453005.

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  CREATE TABLE tpcds.web_returns_p1 ( WR_RETURNED_DATE_SK INTEGER , WR_RETURNED_TIME_SK INTEGER , WR_ITEM_SK INTEGER NOT NULL, WR_REFUNDED_CUSTOMER_SK INTEGER , WR_REFUNDED_CDEMO_SK INTEGER , WR_REFUNDED_HDEMO_SK INTEGER , WR_REFUNDED_ADDR_SK INTEGER , WR_RETURNING_CUSTOMER_SK INTEGER , WR_RETURNING_CDEMO_SK INTEGER , WR_RETURNING_HDEMO_SK INTEGER , WR_RETURNING_ADDR_SK INTEGER , WR_WEB_PAGE_SK INTEGER , WR_REASON_SK INTEGER , WR_ORDER_NUMBER BIGINT NOT NULL, WR_RETURN_QUANTITY INTEGER , WR_RETURN_AMT DECIMAL(7,2) , WR_RETURN_TAX DECIMAL(7,2) , WR_RETURN_AMT_INC_TAX DECIMAL(7,2) , WR_FEE DECIMAL(7,2) , WR_RETURN_SHIP_COST DECIMAL(7,2) , WR_REFUNDED_CASH DECIMAL(7,2) , WR_REVERSED_CHARGE DECIMAL(7,2) , WR_ACCOUNT_CREDIT DECIMAL(7,2) , WR_NET_LOSS DECIMAL(7,2) ) WITH (ORIENTATION = COLUMN,COMPRESSION=MIDDLE) DISTRIBUTE BY HASH (WR_ITEM_SK) PARTITION BY RANGE(WR_RETURNED_DATE_SK) ( PARTITION P1 VALUES LESS THAN(2450815), PARTITION P2 VALUES LESS THAN(2451179), PARTITION P3 VALUES LESS THAN(2451544), PARTITION P4 VALUES LESS THAN(2451910), PARTITION P5 VALUES LESS THAN(2452275), PARTITION P6 VALUES LESS THAN(2452640), PARTITION P7 VALUES LESS THAN(2453005), PARTITION P8 VALUES LESS THAN(MAXVALUE) );

• Example 2: Create a range partitioned table tpcds.web_returns_p2. The table has eight partitions and the data type of their partition key is integer. The upper limit of the eighth partition is MAXVALUE.

  The ranges of the partitions are: wr_returned_date_sk < 2450815, 2450815 ≤ wr_returned_date_sk < 2451179, 2451179 ≤ wr_returned_date_sk < 2451544, 2451544 ≤ wr_returned_date_sk < 2451910, 2451910 ≤ wr_returned_date_sk < 2452275, 2452275 ≤ wr_returned_date_sk < 2452640, 2452640 ≤ wr_returned_date_sk < 2453005, and wr_returned_date_sk ≥ 2453005.

  Assume that CN and DN data directory/pg_location/mount1/path1, CN and DN data directory/pg_location/mount2/path2, CN and DN data directory/pg_location/mount3/path3, and CN and DN data directory/pg_location/mount4/path4 are empty directories for which user dwsadmin has read and write permissions.

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  CREATE TABLE tpcds.web_returns_p2 ( WR_RETURNED_DATE_SK INTEGER , WR_RETURNED_TIME_SK INTEGER , WR_ITEM_SK INTEGER NOT NULL, WR_REFUNDED_CUSTOMER_SK INTEGER , WR_REFUNDED_CDEMO_SK INTEGER , WR_REFUNDED_HDEMO_SK INTEGER , WR_REFUNDED_ADDR_SK INTEGER , WR_RETURNING_CUSTOMER_SK INTEGER , WR_RETURNING_CDEMO_SK INTEGER , WR_RETURNING_HDEMO_SK INTEGER , WR_RETURNING_ADDR_SK INTEGER , WR_WEB_PAGE_SK INTEGER , WR_REASON_SK INTEGER , WR_ORDER_NUMBER BIGINT NOT NULL, WR_RETURN_QUANTITY INTEGER , WR_RETURN_AMT DECIMAL(7,2) , WR_RETURN_TAX DECIMAL(7,2) , WR_RETURN_AMT_INC_TAX DECIMAL(7,2) , WR_FEE DECIMAL(7,2) , WR_RETURN_SHIP_COST DECIMAL(7,2) , WR_REFUNDED_CASH DECIMAL(7,2) , WR_REVERSED_CHARGE DECIMAL(7,2) , WR_ACCOUNT_CREDIT DECIMAL(7,2) , WR_NET_LOSS DECIMAL(7,2) ) DISTRIBUTE BY HASH (WR_ITEM_SK) PARTITION BY RANGE(WR_RETURNED_DATE_SK) ( PARTITION P1 VALUES LESS THAN(2450815), PARTITION P2 VALUES LESS THAN(2451179), PARTITION P3 VALUES LESS THAN(2451544), PARTITION P4 VALUES LESS THAN(2451910), PARTITION P5 VALUES LESS THAN(2452275), PARTITION P6 VALUES LESS THAN(2452640), PARTITION P7 VALUES LESS THAN(2453005), PARTITION P8 VALUES LESS THAN(MAXVALUE) ) ENABLE ROW MOVEMENT;

Links

ALTER TABLE PARTITION, DROP TABLE