CREATE TABLE¶
Function¶
Create an HStore table in the current database. The table will be owned by the user who created it.
In a hybrid data warehouse, you can use DDL statements to create HStore tables. To create an HStore table, set enable_hstore to true and set orientation to column.
Precautions¶
To create an HStore table, you must have the USAGE permission on schema cstore.
The table-level parameters enable_delta and enable_hstore cannot be enabled at the same time. The parameter enable_delta is used to enable delta for common column-store tables and conflicts with enable_hstore.
Each HStore table is bound to a delta table. The OID of the delta table is recorded in the reldeltaidx field in pg_class. (The reldelta field is used by the delta table of the column-store table).
Syntax¶
CREATE TABLE [ IF NOT EXISTS ] table_name
({ column_name data_type
| LIKE source_table [like_option [...] ] }
}
[, ... ])
[ WITH ( {storage_parameter = value} [, ... ] ) ]
[ TABLESPACE tablespace_name ]
[ DISTRIBUTE BY HASH ( column_name [,...])]
[ TO { GROUP groupname | NODE ( nodename [, ... ] ) } ]
[ PARTITION BY {
{RANGE (partition_key) ( partition_less_than_item [, ... ] )}
} [ { ENABLE | DISABLE } ROW MOVEMENT ] ];
The options for LIKE are as follows:
{ INCLUDING | EXCLUDING } { DEFAULTS | CONSTRAINTS | INDEXES | STORAGE | COMMENTS | PARTITION | RELOPTIONS | DISTRIBUTION | ALL }
Differences Between Delta Tables¶
Type | Column-Store Delta Table | HStore Delta Table |
|---|---|---|
Table structure | Same as that defined for the column-store primary table. | Different from that defined for the primary table. |
Function | Used to temporarily store a small batch of inserted data. After the data size reaches the threshold, the data will be merged to the primary table. In this way, data will not be directly inserted to the primary table or generate a large number of small CUs. | Persistently stores UPDATE, DELETE, and INSERT information. It is used to restore the memory structure that manages concurrent updates, such as the memory update chain, in the case of a fault. |
Weakness | If data is not merged in a timely manner, the delta table will grow large and affect query performance. In addition, the table cannot solve lock conflicts during concurrent updates. | The merge operation depends on the background AUTOVACUUM. |
Parameters¶
IF NOT EXISTS
If IF NOT EXISTS is specified, a table will be created if there is no table using the specified name. If there is already a table using the specified name, no error will be reported. A message will be displayed indicating that the table already exists, and the database will skip table creation.
table_name
Specifies the name of the table to be created.
The table name can contain a maximum of 63 characters, including letters, digits, underscores (_), dollar signs ($), and number signs (#). It must start with a letter or underscore (_).
column_name
Specifies the name of a column to be created in the new table.
The column name can contain a maximum of 63 characters, including letters, digits, underscores (_), dollar signs ($), and number signs (#). It must start with a letter or underscore (_).
data_type
Specifies the data type of the column.
LIKE source_table [ like_option ... ]
Specifies a table from which the new table automatically copies all column names and their data types.
The new table and the original table are decoupled after creation is complete. Changes to the original table will not be applied to the new table, and scans on the original table will not be performed on the data of the new table.
Columns copied by LIKE are not merged with the same name. If the same name is specified explicitly or in another LIKE clause, an error will be reported.
HStore tables can be inherited only from HStore tables.
WITH ( { storage_parameter = value } [, ... ] )
Specifies an optional storage parameter for a table.
ORIENTATION
Specifies the storage mode (time series, row-store, or column-store) of table data. This parameter cannot be modified once it is set. For HStore tables, use the column storage mode and set enable_hstore to on.
Options:
TIMESERIES indicates that the data is stored in time series.
COLUMN indicates that the data is stored in columns.
ROW indicates that table data is stored in rows.
Default value: ROW
COMPRESSION
Specifies the compression level of the table data. It determines the compression ratio and time. Generally, a higher compression level indicates a higher compression ratio and a longer compression time, and vice versa. The actual compression ratio depends on the distribution characteristics of loading table data.
Options:
The valid values for HStore tables and 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.
COMPRESSLEVEL
Specifies table data compression rate and duration at the same compression level. This divides a compression level into sub-levels, providing you with more choices for compression ratio and duration. As the value becomes greater, the compression rate becomes higher and duration longer at the same compression level. The parameter is only valid for time series tables and column-store tables.
Value range: 0 to 3
Default value: 0
MAX_BATCHROW
Specifies the maximum number of rows in a storage unit during data loading. The parameter is only valid for time series tables and column-store tables.
Value range: 10000 to 60000
Default value: 60000
PARTIAL_CLUSTER_ROWS
Specifies the number of records to be partially clustered for storage during data loading. The parameter is only valid for time series tables and column-store tables.
Value range: 600000 to 2147483647
enable_delta
Specifies whether to enable delta tables in column-store tables. This parameter cannot be enabled for HStore tables.
Default value: off
enable_hstore
Specifies whether to create a table as an HStore table (based on column-store tables). The parameter is only valid for column-store tables. This parameter is supported by version 8.2.0.100 or later clusters.
Default value: off
Note
If this parameter is enabled, the following GUC parameters must be set to ensure that HStore tables are cleared.
autovacuum=true, autovacuum_max_workers=6, autovacuum_max_workers_hstore=3.
enable_disaster_cstore
Specifies whether fine-grained DR will be enabled for column-store tables. This parameter only takes effect on column-store tables whose COLVERSION is 2.0 and cannot be set to on if enable_hstore is on. This parameter is supported by version 8.2.0.100 or later clusters.
Default value: off
Caution
Before turning on this parameter, set the GUC parameter enable_metadata_tracking to on. Otherwise, the fine-grained DR may fail to be enabled.
SUB_PARTITION_COUNT
Specifies the number of level-2 partitions. This parameter specifies the number of level-2 partitions during data import. This parameter is configured during table creation and cannot be modified after table creation. You are not advised to set the default value, which may affect the import and query performance.
Value range: 1 to 1024
Default value: 32
DELTAROW_THRESHOLD
Specifies the maximum number of rows (SUB_PARTITION_COUNT x DELTAROW_THRESHOLD) to be imported to the delta table.
Value range: 0 to 60000
Default value: 60000
COLVERSION
Specifies the version of the storage format. HStore tables support only version 2.0.
Options:
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
DISTRIBUTE BY
Specifies how the table is distributed or replicated between DNs.
Options:
HASH (column_name): Each row of the table will be placed into all the DNs based on the hash value of the specified column.
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
Specifies the initial partition of an HStore table.
Example¶
Create a simple HStore table.
CREATE TABLE warehouse_t1
(
W_WAREHOUSE_SK INTEGER NOT NULL,
W_WAREHOUSE_ID CHAR(16) NOT NULL,
W_WAREHOUSE_NAME VARCHAR(20) ,
W_WAREHOUSE_SQ_FT INTEGER ,
W_STREET_NUMBER CHAR(10) ,
W_STREET_NAME VARCHAR(60) ,
W_STREET_TYPE CHAR(15) ,
W_SUITE_NUMBER CHAR(10) ,
W_CITY VARCHAR(60) ,
W_COUNTY VARCHAR(30) ,
W_STATE CHAR(2) ,
W_ZIP CHAR(10) ,
W_COUNTRY VARCHAR(20) ,
W_GMT_OFFSET DECIMAL(5,2)
)WITH(ORIENTATION=COLUMN, ENABLE_HSTORE=ON);
CREATE TABLE warehouse_t2 (LIKE warehouse_t1 INCLUDING ALL);