Creating and Managing Tables

Creating a Table

You can run the CREATE TABLE command to create a table. When creating a table, you can define the following information:

• Columns and data type of the table.
• Table or column constraints that restrict a column or the data contained in a table. For details, see Definition of Table Constraints.
• Distribution policy of a table, which determines how the GaussDB (DWS) database divides data between segments. For details, see Definition of Table Distribution.
• Table storage format. For details, see Selecting a Table Storage Mode.
• Partition table information. For details, see Defining Table Partitions.

Definition of Table Constraints

You can define constraints on columns and tables to restrict data in a table. However, there are the following restrictions:

• The primary key constraint and unique constraint in the table must contain a distribution column.
• 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.

1
2
3
4
5
6

CREATE TABLE products
(
   product_no integer, 
   name text, 
   price numeric CHECK (price > 0)
);

1
2
3
4
5
6

CREATE TABLE products 
( 
    product_no integer NOT NULL,
    name text NOT NULL,
    price numeric 
);

1
2
3
4
5
6

CREATE TABLE products 
(
    product_no integer UNIQUE, 
    name text, 
    price numeric
)DISTRIBUTE BY HASH(product_no);

1
2
3
4
5
6

CREATE TABLE products 
( 
    product_no integer PRIMARY KEY, 
    name text, 
    price numeric
)DISTRIBUTE BY HASH(product_no);

1
2
3
4
5
6
7

CREATE TABLE products 
(
    product_no integer, 
    name text, 
    price numeric,
    PARTIAL CLUSTER KEY(product_no)
) WITH (ORIENTATION = COLUMN);

Definition of Table Distribution

GaussDB(DWS) supports the following distribution modes: replication, hash, and roundrobin.

The roundrobin distribution mode is supported only by cluster version 8.1.2 or later.

Policy	Description	Scenario	Advantages/Disadvantages
Replication	Full data in a table is stored on each DN in the cluster.	Small tables and dimension tables	The advantage of replication is that each DN has full data of the table. During the join operation, data does not need to be redistributed, reducing network overheads and reducing plan segments (each plan segment starts a corresponding thread). The disadvantage of replication is that each DN retains the complete data of the table, resulting in data redundancy. Generally, replication is only used for small dimension tables.
Hash	Table data is distributed on all DNs in the cluster.	Fact tables containing a large amount of data	The I/O resources of each node can be used during data read/write, greatly improving the read/write speed of a table. Generally, a large table (containing over 1 million records) is defined as a hash table.
Polling (Round-robin)	Each row in the table is sent to each DN in turn. Data can be evenly distributed on each DN.	Fact tables that contain a large amount of data and cannot find a proper distribution column in hash mode	Round-robin can avoid data skew, improving the space utilization of the cluster. Round-robin does not support local DN optimization like a hash table does, and the query performance of Round-robin is usually lower than that of a hash table. If a proper distribution column can be found for a large table, use the hash distribution mode with better performance. Otherwise, define the table as a round-robin table.

View the data in the table.

• Run the following command to query information about all tables in a database in the system catalog pg_tables:

  1	SELECT * FROM pg_tables;

• Run the \d+ command of the gsql tool to query table attributes:

  1	\d+ customer_t1;

• Run the following command to query the data volume of table customer_t1:

  1	SELECT count(*) FROM customer_t1;

• Run the following command to query all data in table customer_t1:

  1	SELECT * FROM customer_t1;

• Run the following command to query data in column c_customer_sk:

  1	SELECT c_customer_sk FROM customer_t1;

• Run the following command to filter repeated data in column c_customer_sk:

  1	SELECT DISTINCT( c_customer_sk ) FROM customer_t1;

• Run the following command to query all data whose column c_customer_sk is 3869:

  1	SELECT * FROM customer_t1 WHERE c_customer_sk = 3869;

• Run the following command to sort data based on column c_customer_sk.

  1	SELECT * FROM customer_t1 ORDER BY c_customer_sk;

Deleting Data in a Table

Exercise caution when running the DROP TABLE and TRUNCATE TABLE statements. After a table is deleted, data cannot be restored.

• Delete the customer_t1 table from the database.

  1	DROP TABLE customer_t1;

• You can use DELETE or TRUNCATE to clear rows in a table without removing the definition of the table.
  Delete all rows from the customer_t1 table.

  1	TRUNCATE TABLE customer_t1;

  Delete all rows from the customer_t1 table.

  1	DELETE FROM customer_t1;

  Delete all records whose c_customer_sk is 3869 from the customer_t1 table.

  1	DELETE FROM customer_t1 WHERE c_customer_sk = 3869;