CREATE TABLE¶
Function¶
Creates a new empty table in the current database.
This table is owned by the user who executes the command. However, if the system administrator creates a table in the schema with the same name as a common user, the owner of the table is the user (not the system administrator).
Precautions¶
For details about the data types supported by column-store tables, see Data Types Supported by Column-Store Tables.
It is recommended that the number of column-store and HDFS partitioned tables do not exceed 1000.
The primary key constraint and unique constraint in the table must contain a distribution column.
The data type of the distribution column in an existing table cannot be modified.
A system column cannot be designated as a primary key in a row-store REPLICATION distributed table.
If an error occurs during table creation, after it is fixed, the system may fail to delete the empty disk files created before the last automatic clearance. This problem seldom occurs.
Column-store tables support the PARTIAL CLUSTER KEY and table-level primary key and unique constraints, but do not support table-level foreign key constraints.
Only the NULL, NOT NULL, and DEFAULT constant values can be used as column-store table column constraints.
Whether column-store tables support a delta table is specified by the enable_delta parameter. The threshold for storing data into a delta table is specified by the deltarow_threshold parameter. Using column-store tables with delta tables is not recommended. This may cause disk bloat and performance deterioration due to delayed merge.
Multi-temperature tables support only partitioned column-store tables and depend on available OBS tablespaces.
Multi-temperature tables support only the default tablespace default_obs_tbs. If you need to add an OBS tablespace, contact technical support.
Syntax¶
CREATE [ [ GLOBAL | LOCAL | VOLATILE ] { TEMPORARY | TEMP } | UNLOGGED ] TABLE [ IF NOT EXISTS ] table_name
{ ({ column_name data_type [ compress_mode ] [ COLLATE collation ] [ column_constraint [ ... ] ]
| table_constraint
| LIKE source_table [ like_option [...] ] }
[, ... ])|
LIKE source_table [ like_option [...] ] }
[ WITH ( {storage_parameter = value} [, ... ] ) ]
[ ON COMMIT { PRESERVE ROWS | DELETE ROWS ]
[ COMPRESS | NOCOMPRESS ]
[ DISTRIBUTE BY { REPLICATION | ROUNDROBIN | { HASH ( column_name [,...] ) } } ]
[ TO { GROUP groupname | NODE ( nodename [, ... ] ) } ]
[ COMMENT [=] 'text' ];
column_constraint is as follows:
[ CONSTRAINT constraint_name ] { NOT NULL | NULL | CHECK ( expression ) | DEFAULT default_expr | ON UPDATE on_update_expr | COMMENT 'text' | UNIQUE [ NULLS [NOT] DISTINCT | NULLS IGNORE ] index_parameters | PRIMARY KEY index_parameters } [ DEFERRABLE | NOT DEFERRABLE | INITIALLY DEFERRED | INITIALLY IMMEDIATE ]
compress_mode of a column is as follows:
{ DELTA | PREFIX | DICTIONARY | NUMSTR | NOCOMPRESS }
table_constraint is as follows:
[ CONSTRAINT constraint_name ] { CHECK ( expression ) | UNIQUE [ NULLS [NOT] DISTINCT | NULLS IGNORE ] ( column_name [, ... ] ) index_parameters | PRIMARY KEY ( column_name [, ... ] ) index_parameters | PARTIAL CLUSTER KEY ( column_name [, ... ] ) } [ DEFERRABLE | NOT DEFERRABLE | INITIALLY DEFERRED | INITIALLY IMMEDIATE ]
like_option is as follows:
{ INCLUDING | EXCLUDING } { DEFAULTS | CONSTRAINTS | INDEXES | STORAGE | COMMENTS | PARTITION | RELOPTIONS | DISTRIBUTION | DROPCOLUMNS | ALL }
index_parameters is as follows:
[ WITH ( {storage_parameter = value} [, ... ] ) ]
Parameters¶
UNLOGGED
If this key word is specified, the created table is not a log table. Data written to unlogged tables is not written to the write-ahead log, which makes them considerably faster than ordinary tables. However, an unlogged table is automatically truncated after a crash or unclean shutdown, incurring data loss risks. The contents of an unlogged table are also not replicated to standby servers. Any indexes created on an unlogged table are not automatically logged as well.
Usage scenario: Unlogged tables do not ensure safe data. Users can back up data before using unlogged tables; for example, users should back up the data before a system upgrade.
Troubleshooting: If data is missing in the indexes of unlogged tables due to some unexpected operations such as an unclean shutdown, users should re-create the indexes with errors.
Important
The UNLOGGED table uses no primary/standby mechanism. In the case of system faults or abnormal breakpoints, data loss may occur. Therefore, the UNLOGGED table cannot be used to store basic data.
GLOBAL | LOCAL | VOLATILE
Specify the keywords GLOBAL, LOCAL, and VOLATILE before TEMP or TEMPORARY to create temporary tables with different attributes.
Currently, the keywords GLOBAL and LOCAL are introduced for standard SQL compatibility. No matter whether GLOBAL or LOCAL is specified, the GaussDB(DWS) creates a LOCAL temporary table.
If VOLATILE is specified, a temporary volatile table is created.
TEMPORARY | TEMP
If TEMP or TEMPORARY is specified, the created table is a temporary table. Temporary tables are automatically dropped at the end of a session, or optionally at the end of the current transaction. Therefore, apart from CN and other CN errors connected by the current session, you can still create and use temporary table in the current session. Temporary tables are created only in the current session. If a DDL statement involves operations on temporary tables, a DDL error will be generated. Therefore, you are not advised to perform operations on temporary tables in DDL statements. TEMP is equivalent to TEMPORARY.
Important
Local or volatile temporary tables are visible to the current session through schema of the pg_temp start. Users should not delete schema started with pg_temp, pg_toast_temp.
If TEMPORARY or TEMP is not specified when you create a table and the schema of the specified table starts with pg_temp_, the table is created as a temporary table.
Similar to common tables, all metadata of local temporary tables is stored in system catalogs. Volatile temporary tables store table structure metadata in memory, except the schema metadata. Compared with local temporary tables, volatile temporary tables have the following constraints:
After a CN or DN is restarted, data in its memory will be lost, and accordingly, volatile temporary tables on it will become invalid.
Currently, volatile temporary tables do not support table structure modification, such as ALTER and GRANT.
Volatile temporary tables and local temporary tables share temporary schemas. Therefore, in the same session, the VOLATILE temporary table and local temporary table cannot have the same name.
Volatile temporary table information is not stored in system catalogs. Therefore, Volatile metadata cannot be queried by running DML statements in system catalogs.
Volatile temporary tables support only common row-store and column-store tables. Delta tables, time series tables, and cold and hot tables are not supported.
Views cannot be created based on volatile temporary tables.
A tablespace cannot be specified when a temporary table is created. The default tablespace of a volatile temporary table is pg_volatile.
The following constraints cannot be specified when a volatile temporary table is created: CHECK, UNIQUE, PRIMARY KEY, TRIGGER, EXCLUDE, and PARTIAL CLUSTER.
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 ($), and number signs (#). It must start with a letter or underscore (_).
A table name enclosed in double quotation marks can contain spaces and special characters. However, you are not advised to use these characters in a table name because they may make it difficult to reference and use. In addition, they may be processed differently under different database compatibility modes.
column_name
Specifies the name of a column to be created in the new table.
The column name can contain a maximum of 63 characters, including letters, digits, underscores (_), dollar signs ($), and number signs (#). It must start with a letter or underscore (_).
data_type
Specifies the data type of the column.
Note
In a database that is compatible with Teradata or MySQL syntax, if the data type of a column is set to DATE, the DATE type is returned. Otherwise, the TIMESTAMP type is returned.
compress_mode
Specifies the compress option of the table, only available for row-store table. The option specifies the algorithm preferentially used by table columns.
Value range: DELTA, PREFIX, DICTIONARY, NUMSTR, NOCOMPRESS
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.
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.
The new table and the source table are decoupled after creation is complete. Changes to the source 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 source table.
Columns and constraints copied by LIKE are not merged with the same name. If the same name is specified explicitly or in another LIKE clause, an error is reported.
The default expressions or the ON UPDATE expressions are copied from the source table to the new table only 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.
The CHECK constraints are copied from the source table to the new table only when INCLUDING CONSTRAINTS is specified. Other types of constraints are never copied to the new table. NOT NULL constraints are always copied to the new table. These rules also apply to column constraints and table constraints.
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 are copied only if INCLUDING STORAGE is specified. The default behavior is to exclude STORAGE settings.
If INCLUDING COMMENTS is specified, comments for the copied columns, constraints, and indexes are copied. The default behavior is to exclude comments.
If INCLUDING PARTITION is specified, the partition definitions of the source table are copied to the new table, and the new table no longer uses the PARTITION BY clause. The default behavior is to exclude partition definition of the source table.
If INCLUDING RELOPTIONS is specified, the storage parameter (WITH clause of the source table) of the source table is copied to the new table. The default behavior is to exclude partition definition of the storage parameter of the source table.
Note
PERIOD and TTL in the WITH clause are partition-related parameters. LIKE INCLUDING RELOPTIONS will not be copied to the new table. To copy LIKE INCLUDING RELOPTIONS, use INCLUDING PARTITION.
If INCLUDING DISTRIBUTION is specified, the distribution information of the source table is copied to the new table, including distribution type and column, and the new table no longer use the DISTRIBUTE BY clause. The default behavior is to exclude distribution information of the source table.
If INCLUDING DROPCOLUMNS is specified, the deleted column information in the source table is copied to the new table. By default, the deleted column information of the source table is not copied.
INCLUDING ALL contains the meaning of INCLUDING DEFAULTS, INCLUDING CONSTRAINTS, INCLUDING INDEXES, INCLUDING STORAGE, INCLUDING COMMENTS, INCLUDING PARTITION, INCLUDING RELOPTIONS, INCLUDING DISTRIBUTION, and INCLUDING DROPCOLUMNS.
If EXCLUDING is specified, the specified parameters are not included.
For an OBS multi-temperature table, all partitions of the new table are local hot partitions after INCLUDING PARTITION is specified.
Important
If the source table contains a sequence with the SERIAL, BIGSERIAL, or SMALLSERIAL data type, or a column in the source table is a sequence by default and the sequence is created for this table by using CREATE SEQUENCE... OWNED BY, these sequences will not be copied to the new table, and another sequence specific to the new table will be created. This is different from earlier versions. To share a sequence between the source table and new table, create a shared sequence (do not use OWNED BY) and set a column in the source table to this sequence.
You are not advised to set a column in the source table to the sequence specific to another table especially when the table is distributed in specific Node Groups, because doing so may result in CREATE TABLE ... LIKE execution failures. In addition, doing so may cause the sequence to become invalid in the source sequence because the sequence will also be deleted from the source table when it is deleted from the table that the sequence is specific to. To share a sequence among multiple tables, you are advised to create a shared sequence for them.
WITH ( { storage_parameter = value } [, ... ] )
Specifies an optional storage parameter for a table or an index.
Note
Using Numeric of any precision to define column, specifies precision p and scale s. When precision and scale are not specified, the input will be displayed.
The description of parameters is 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 to 100
ORIENTATION
Specifies the storage mode (row-store, column-store) for table data. This parameter cannot be modified once it is set.
Valid value:
ROW indicates that table data is stored in rows.
ROW applies to OLTP service, which has many interactive transactions. An interaction involves many columns in the table. Using ROW can improve the efficiency.
COLUMN indicates that the data is stored in columns.
COLUMN applies to the data warehouse service, which has a large amount of aggregation computing, and involves a few column operations.
Default value: ROW (row-store)
Note
In cluster 8.1.3 and later versions, the GUC parameter default_orientation (default value: row) is added. If the storage mode is not specified when a table is created, by default, the table is created based on the value of the parameter (row, column, column enabledelta).
COMPRESSION
Specifies the compression level of the table data. It determines the compression ratio and time. Generally, the higher the level of compression, the higher the ratio, the longer the time, and the lower the level of compression, the lower the ratio, the shorter the time. The actual compression ratio depends on the distribution characteristics of loading table data.
Valid value:
The valid values for column-store tables are YES/NO and LOW/MIDDLE/HIGH, and the default is LOW. If this parameter is set to YES, the compression level is LOW by default.
Note
Currently, compression is unavailable for row-store tables.
To determine the size of a new GaussDB(DWS) cluster, consider the size of ORC data compressed and migrated to column-store tables in GaussDB(DWS). If the compression level is low, the size of a copy is about 1.5 to 2 times that of ORC. If the compression level is high, the size of a copy is basically the same as that of ORC.
The middle compression of column-stores uses dictionary compression. For data not suitable for dictionary compression, the file size after middle compression may be greater than that of after low compression.
GaussDB(DWS) provides the following compression algorithms:
Table 1 Compression algorithms for column-based storage¶ COMPRESSION
NUMERIC
STRING
INT
LOW
Delta compression + RLE compression
LZ4 compression
Delta compression (RLE is optional.)
MIDDLE
Delta compression + RLE compression + LZ4 compression
dict compression or LZ4 compression
Delta compression or LZ4 compression (RLE is optional)
HIGH
Delta compression + RLE compression + zlib compression
dict compression or zlib compression
Delta compression or zlib compression (RLE is optional)
COMPRESSLEVEL
Specifies the compression level of the table data. It determines the compression ratio and time. This divides a compression level into sublevels, providing you with more choices for compression rate 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 column-store tables.
Value range: 0-3.
Default value: 0
TTL
Schedules the partition deletion tasks in a partitioned table. By default, no partition deletion task is created.
Value range: 1 hour-100 years
PERIOD
Schedules the partition creation tasks in a partitioned table. If TTL has been configured, PERIOD cannot be greater than TTL.
Value range: 1 hour-100 years
Default value: 1 day
MAX_BATCHROW
Specifies the maximum of a storage unit during data loading process. The parameter is only valid for column-store tables.
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 tables.
Value range: 600000 to 2147483647
enable_delta
Specifies whether to enable delta tables in column-store tables. The parameter is only valid for column-store tables.
Default value: off
enable_hstore
Specifies whether an H-Store table will be created (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 H-Store tables are cleared.
autovacuum=on, 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 true if enable_hstore is true. This parameter is supported by version 8.2.0.100 or later clusters.
fine_disaster_table_role
This parameter has been discarded in version 8.2.1 and is reserved for compatibility with earlier versions. This parameter is invalid in the current version.
Specifies whether the fine-grained DR table will be set as a primary or secondary table. This parameter can be true only when the enable_disaster_cstore parameter has been set to true.
Valid value:
primary: Specifies the primary fine-grained DR table.
standby: Specifies the standby fine-grained DR table.
DELTAROW_THRESHOLD
Specifies the upper limit of to-be-imported rows for triggering the data import to a delta table when data is to be imported to a column-store table. This parameter takes effect only if the enable_delta table parameter is set to on. The parameter is only valid for column-store tables.
Value range: 0 to 60000
Default value: 6000
COLVERSION
Specifies the version of the column-store format. You can switch between different storage formats.
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 multi-temperature tables.
Note
For clusters of version 8.1.0, the default value of this parameter is 1.0. For clusters of version 8.1.1 or later, the default value of this parameter is 2.0. If the cluster version is upgraded from 8.1.0 to 8.1.1 or later, the default value of this parameter changes from 1.0 to 2.0.
When creating a column-store table, set COLVERSION to 2.0. Compared with the 1.0 storage format, the performance is significantly improved:
The time required for creating a column-store wide table is significantly reduced.
In the Roach data backup scenario, the backup time is significantly reduced.
The build and catch up time is greatly reduced.
The occupied disk space decreases significantly.
analyze_mode
Specifies the mode of table-level auto-analyze.
Valid value:
frozen: disables all ANALYZE operations (dynamic sampling can still be triggered when no statistics are collected).
backend: allows only ANALYZE triggered by AUTOVACUUM polling.
runtime: allows only runtime ANALYZE triggered by the optimizer.
all: Both backend and runtime AUTO-ANALYZE can be triggered.
Default value: all
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
Note
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.
enable_column_autovacuum_garbage
Determines whether to enable CU rewriting logic for column-store tables using AUTOVACUUM. This parameter is supported only by clusters of version 8.2.1.100 or later.
There is a low probability that an error is reported when lightweight UPDATE and AUTOVACUUM are executed concurrently. You can set the table-level parameter to off to avoid this problem.
Default value: true
ON COMMIT { PRESERVE ROWS | DELETE ROWS }
ON COMMIT determines what to do when you commit a temporary table creation operation.
PRESERVE ROWS (Default): No special action is taken at the ends of transactions. The temporary table and its table data are unchanged.
DELETE ROWS: All rows in the temporary table will be deleted at the end of each transaction block.
COMPRESS | NOCOMPRESS
If you specify COMPRESS in the CREATE TABLE statement, 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.
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.
ROUNDROBIN: Each row in the table is sent to each DN in turn. Therefore, data is evenly distributed on each DN. This value is supported only in 8.1.2 or later.
HASH (column_name): Each row of the table will be placed into all the DNs based on the hash value of the specified column.
Note
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.
If TO GROUP is set to a replication table node group (supported in 8.1.2 or later), DISTRIBUTE BY must be set to REPLICATION. If DISTRIBUTE BY is not specified, the created table is automatically set as a replication table.
The hybrid data warehouse (standalone) has only one DN. Therefore, the distribution rule is ignored and cannot be modified.
Default value: determined by the GUC parameter default_distribution_mode
When default_distribution_mode is set to roundrobin, the default value of DISTRIBUTE BY is selected according to the following rules:
If the primary key or unique constraint is included during table creation, hash distribution is selected. The distribution column is the column corresponding to the primary key or unique constraint.
If the primary key or unique constraint is not included during table creation, round-robin distribution is selected.
When default_distribution_mode is set to hash, the default value of DISTRIBUTE BY is selected according to the following rules:
If the primary key or unique constraint is included during table creation, hash distribution is selected. The distribution column is the column corresponding to the primary key or unique constraint.
If the primary key or unique constraint is not included during table creation but there are columns whose data types can be used as distribution columns, hash distribution is selected. The distribution column is the first column whose data type can be used as a distribution column.
If the primary key or unique constraint is not included during table creation and no column whose data type can be used as a distribution column exists, round-robin distribution is selected.
The following data types can be used as distribution columns:
Integer types: TINYINT, SMALLINT, INT, BIGINT, and NUMERIC/DECIMAL
Character types: CHAR, BPCHAR, VARCHAR, VARCHAR2, NVARCHAR2, and TEXT
Date/time types: DATE, TIME, TIMETZ, TIMESTAMP, TIMESTAMPTZ, INTERVAL, and SMALLDATETIME
Note
When you create a table, the choices of distribution keys and partition keys have major impact on SQL query performance. Therefore, choosing proper distribution column and partition key with strategies.
Selecting an Appropriate Distribution Column
In the data distributed table using Hash, an appropriate distributed array should be used to distribute and store data on multiple DNs evenly, preventing data skew (uneven data distribution across several DNs). Determine the proper distribution column based on the following principles:
Determine whether data is skewed.
Connect to the database and run the following statements to check the number of tuples on each DN: Replace tablename with the actual name of the table to be analyzed.
SELECT a.count,b.node_name FROM (SELECT count(*) AS count,xc_node_id FROM tablename GROUP BY xc_node_id) a, pgxc_node b WHERE a.xc_node_id=b.node_id ORDER BY a.count DESC;
If tuple numbers vary greatly (several times or tenfold) in each DN, a data skew occurs. Change the data distribution key based on the following principles:
Run the ALTER TABLE statement to adjust the distribution column. The rules for selecting a distribution column are as follows:
The column value of the distribution column should be discrete so that data can be evenly distributed on each DN. For example, you are advised to select the primary key of a table as the distribution column, and the ID card number as the distribution column in a personnel information table.
With the above principles met, you can select join conditions as distribution keys so that join tasks can be pushed down to DNs, reducing the amount of data transferred between the DNs.
If a proper distribution column cannot be found to make data evenly distributed on each DN, you can use the REPLICATION or ROUNDROBIN data distribution mode. The REPLICATION data distribution mode stores complete data on each DN. Therefore, if a table is large and no proper distribution column can be found, the ROUNDROBIN data distribution mode is recommended. The ROUNDROBIN data distribution mode is supported in 8.1.2 or later.
Selecting appropriate partition keys
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.
Modify partition keys to make the query result stored in the same or least partitions (partition pruning). Obtaining consecutive I/O to improve the query performance.
In actual services, time is used to filter query objects. Therefore, you can use time as a partition key, and change the key value based on the total data volume and single data query volume.
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.
In logical cluster mode, if TO GROUP is not specified, the table is created in the node group associated with the logical cluster user by default. If the user, such as the administrator or a common user, does not manage the logical cluster, by default the table is created in the first logical cluster, which is the logical cluster with the smallest OID in pgxc_group.
If the node group specified by TO GROUP is a replication table node group, the table is created on all CNs and DNs, but the replication table data is distributed only on the DNs in the replication table node group.
COMMENT [=] 'text'
The COMMENT clause can specify table comments during table creation.
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.
NOT NULL
Indicates that the column is not allowed to contain NULL values.
NULL
The column is allowed to contain NULL values. This is the default setting.
This clause is only provided for compatibility with non-standard SQL databases. You are advised not to use this clause.
CHECK ( expression )
Specifies an expression producing a Boolean result which new or updated rows must satisfy for an insert or update operation to succeed. Expressions evaluating to TRUE or UNKNOWN succeed. If any row of an insert or update operation produces a FALSE result, an error exception is raised and the insert or update does not alter the database.
A check constraint specified as a column constraint should reference only the column's values, while an expression appearing in a table constraint can reference multiple columns.
Note
<>NULL and !=NULL are invalid in an expression. Change them to IS NOT NULL.
DEFAULT default_expr
Assigns a default data value for a column. The value can be any variable-free expressions (Subqueries and cross-references to other columns in the current table are not allowed). The data type of the default expression must match the data type of the column.
The default expression will be used in any insert operation that does not specify a value for the column. If there is no default value for a column, then the default value is NULL.
ON UPDATE on_update_expr
The ON UPDATE clause specifies a timestamp function for a column. Ensure that the data type of the column for which the ON UPDATE clause specifies a timestamp function is timestamp or timestamptz.
When an SQL statement containing the UPDATE operation is executed, this column is automatically updated to the time specified by the timestamp function.
Note
The on_update_expr function supports only CURRENT_TIMESTAMP, CURRENT_TIME, CURRENT_DATE, LOCALTIME, LOCALTIMESTAMP.
COMMENT 'text'
The COMMENT clause can specify a comment for a column.
UNIQUE [ NULLS [ NOT ] DISTINCT | NULLS IGNORE ] index_parameters
UNIQUE [ NULLS [ NOT ] DISTINCT | NULLS IGNORE ] ( column_name [, ... ] ) index_parameters
Specifies that a group of one or more columns of a table can contain only unique values.
The [ NULLS [ NOT ] DISTINCT | NULLS IGNORE ] field is used to specify how to process null values in the index column of the Unique index.
Default value: This parameter is left empty by default. NULL values can be inserted repeatedly.
When the inserted data is compared with the original data in the table, the NULL value can be processed in any of the following ways:
NULLS DISTINCT: NULL values are unequal and can be inserted repeatedly.
NULLS NOT DISTINCT: NULL values are equal. If all index columns are NULL, NULL values cannot be inserted repeatedly. If some index columns are NULL, data can be inserted only when non-null values are different.
NULLS IGNORE: NULL values are skipped during the equivalent comparison. If all index columns are NULL, NULL values can be inserted repeatedly. If some index columns are NULL, data can be inserted only when non-null values are different.
The following table lists the behaviors of the three processing modes.
Table 2 Processing of NULL values in index columns in unique indexes¶ Constraint
All Index Columns Are NULL
Some Index Columns Are NULL.
NULLS DISTINCT
Can be inserted repeatedly.
Can be inserted repeatedly.
NULLS NOT DISTINCT
Cannot be inserted repeatedly.
Cannot be inserted if the non-null values are equal. Can be inserted if the non-null values are not equal.
NULLS IGNORE
Can be inserted repeatedly.
Cannot be inserted if the non-null values are equal. Can be inserted if the non-null values are not equal.
Note
If DISTRIBUTE BY REPLICATION is not specified, the column table that contains only unique values must contain distribution columns.
PRIMARY KEY index_parameters
PRIMARY KEY ( column_name [, ... ] ) index_parameters
Specifies the primary key constraint specifies that a column or columns of a table can contain only unique (non-duplicate) and non-null values.
Only one primary key can be specified for a table.
Note
If DISTRIBUTE BY REPLICATION is not specified, the column set with a primary key constraint must contain distributed columns.
DEFERRABLE | NOT DEFERRABLE
Controls whether the constraint can be deferred. A constraint that is not deferrable will be checked immediately after every command. Checking of constraints that are deferrable can be postponed until the end of the transaction using the SET CONSTRAINTS command. NOT DEFERRABLE is the default value. Currently, only UNIQUE and PRIMARY KEY constraints of row-store tables accept this clause. All the other constraints are not deferrable.
PARTIAL CLUSTER KEY
Specifies a partial cluster key for storage. When importing data to a column-store table, you can perform local data sorting by specified columns (single or multiple).
INITIALLY IMMEDIATE | INITIALLY DEFERRED
If a constraint is deferrable, this clause specifies the default time to check the constraint.
If the constraint is INITIALLY IMMEDIATE (default value), it is checked after each statement.
If the constraint is INITIALLY DEFERRED, it is checked only at the end of the transaction.
The constraint check time can be altered using the SET CONSTRAINTS command.
Examples¶
Define a unique column constraint for the table:
CREATE TABLE CUSTOMER
(
C_CUSTKEY BIGINT NOT NULL CONSTRAINT C_CUSTKEY_pk PRIMARY KEY ,
C_NAME VARCHAR(25) ,
C_ADDRESS VARCHAR(40) ,
C_NATIONKEY INT ,
C_PHONE CHAR(15) ,
C_ACCTBAL DECIMAL(15,2)
)
DISTRIBUTE BY HASH(C_CUSTKEY);
Define a primary key table constraint for the table. You can define a primary key table constraint on one or more columns of a table:
CREATE TABLE CUSTOMER
(
C_CUSTKEY BIGINT ,
C_NAME VARCHAR(25) ,
C_ADDRESS VARCHAR(40) ,
C_NATIONKEY INT ,
C_PHONE CHAR(15) ,
C_ACCTBAL DECIMAL(15,2) ,
CONSTRAINT C_CUSTKEY_KEY PRIMARY KEY(C_CUSTKEY,C_NAME)
)
DISTRIBUTE BY HASH(C_CUSTKEY,C_NAME);
Define the CHECK column constraint.
CREATE TABLE CUSTOMER
(
C_CUSTKEY BIGINT NOT NULL CONSTRAINT C_CUSTKEY_pk PRIMARY KEY ,
C_NAME VARCHAR(25) ,
C_ADDRESS VARCHAR(40) ,
C_NATIONKEY INT NOT NULL CHECK (C_NATIONKEY > 0)
)
DISTRIBUTE BY HASH(C_CUSTKEY);
Define the CHECK table constraint:
CREATE TABLE CUSTOMER
(
C_CUSTKEY BIGINT NOT NULL CONSTRAINT C_CUSTKEY_pk PRIMARY KEY ,
C_NAME VARCHAR(25) ,
C_ADDRESS VARCHAR(40) ,
C_NATIONKEY INT ,
CONSTRAINT C_CUSTKEY_KEY2 CHECK(C_CUSTKEY > 0 AND C_NAME <> '')
)
DISTRIBUTE BY HASH(C_CUSTKEY);
Create a column-store table and specify the storage format and compression mode:
CREATE TABLE customer_address
(
ca_address_sk INTEGER NOT NULL ,
ca_address_id CHARACTER(16) NOT NULL ,
ca_street_number CHARACTER(10) ,
ca_street_name CHARACTER varying(60) ,
ca_street_type CHARACTER(15) ,
ca_suite_number CHARACTER(10)
)
WITH (ORIENTATION = COLUMN, COMPRESSION=HIGH,COLVERSION=2.0)
DISTRIBUTE BY HASH (ca_address_sk);
Use DEFAULT to declare a default value for column W_STATE:
CREATE TABLE warehouse_t
(
W_WAREHOUSE_SK INTEGER NOT NULL,
W_WAREHOUSE_ID CHAR(16) NOT NULL,
W_WAREHOUSE_NAME VARCHAR(20) UNIQUE DEFERRABLE,
W_WAREHOUSE_SQ_FT INTEGER ,
W_COUNTY VARCHAR(30) ,
W_STATE CHAR(2) DEFAULT 'GA',
W_ZIP CHAR(10)
);
Create the CUSTOMER_bk table in LIKE mode:
CREATE TABLE CUSTOMER_bk (LIKE CUSTOMER INCLUDING ALL);