Kingbasees plug-in KDB of Jincang database_ database_ link

1.Database Link Introduce

Database Link abbreviation DBLink, It is a mechanism provided by database management system for accessing external database objects . Used to pass through DBLink To access tables in an external database 、 View objects .DBLink Realize the communication between two databases .DBLink It includes a network-based data connection and the user name used to log in to the remote database 、 Password information . The local database system can be accessed through DBLink Establish a session with the remote database , So as to complete the access to remote database objects .DBLink Realized SQL/MED Some functions defined in the standard to access external data sources .

  Note:

SQL/MED yes SQL Management of External Data Abbreviation , yes ISO/IEC 9075-9:2003 Standard middle right SQL Language extension .SQL/MED The standard specifies how to use an external data wrapper （Foreign Data Wrapper） And data connection （datalink） Achieve access to external data . External data here refers to data that can be based on SQL Of DBMS Data accessed .

dblink Object is a database internal object , Can pass dblink Object connection refers to objects in other databases . Support connection Oracle、Kingbase and Postgresql Of dblink object .

Related definitions are as follows ：

• Local database ： The database to which the current database client is connected , You can directly access and operate objects with corresponding permissions in this database , The client holds the connection .

• Remote database ： By defining dblink Database accessed , Although the client seems to have direct access to the objects of the remote database , In fact, such connections are held by the local database , System information such as object definition does not exist in the local database . The remote database may just be connected to another service process or thread of its own by setting the local database .

• Remote connection ： adopt dblink Database connection created when accessing remote database objects , For the target database , The local database is the client for which it provides services . The local database has a remote connection to itself , Need reasonable management , Close it in time when necessary , Prevent excessive resource overhead .

• remote table （ View 、 Materialized view ）： Tables that exist on the remote database , It can also be views or materialized views , The local database doesn't care about this , Only care about the definition of the obtained data structure and the data itself .

• Remote sequence ： Sequence generators that exist on remote databases , Usually used to unify id Generation .

• Remote functions （ stored procedure ）： Functions or stored procedures that exist on remote databases , In general, there are many scenarios where functions are called remotely .

• Remote synonyms ： Exists in the local database pointing to remote objects （ surface 、 Sequence 、 Function, etc. ） A synonym for .

2. Database Link function

Dblink The function is mainly to meet the adaptation of common grammar , Let the code applied by users be applicable to a wider range of products without massive modification during migration ：

• Support connection management , Close the connection at the appropriate time to reduce the resource overhead of the remote database .

• Support remote tables （ View 、 Materialized view ） Query for , And support push down query conditions to reduce the network overhead of data transmission .

• Support remote table insertion .

• Support remote sequence access .

3.  establish Database Link object

Creating dblink Object to specify connection information , Include remote database network address 、 port 、 Database name 、 User name and password , What specific information needs to be provided will vary from database to database .

Grammar format ：

CREATE [ PUBLIC ] DATABASE LINK dblink
       CONNECT TO user IDENTIFIED BY password USING
       {(connect_string) | config_tag };

Parameter description ：

PUBLIC

Appoint PUBLIC Create public dblink object , All users can access public dblink object . public dblink Is created in PUBLIC Under Mode . Not specified PUBLIC Create private dblink, Don't be in PUBLIC Create private dblink.

dblink

Specify the name of the database link to create , In the same mode dblink You can't have the same name .

user IDENTIFIED BY password

Specify the user name and password of the remote database .

connect_string

Configure connection string information ,connect_string Used to provide information for connecting to a remote database .

config_tag

Configuration file tag name , Used to indicate the profile （sys_database_link.conf） One of them , Thus, the network address of the remote database used to establish the database connection can be obtained through the configuration file 、 Port and database name .

annotation ：

1、 The format in the configuration file is as follows ：

[ name ]

DriverName= Connection driver name .

Host= Remote database network address .

Port= Remote database service port .

Dbname= Remote database name .

DbType= Remote database type .

DbType Support Oracle,Kingbase,Postgres There are three database types .

2、 When using this function , Need to kdb_database_link Join in shared_preload_libraries in .

3、 When querying the external database , You need to create the corresponding database plug-in , Such as :kingbase_fdw、oracle_fdw、postgres_fdw.

give an example ：

Create one to Oracle Database connection of database , It can be used by all database users ：

CREATE PUBLIC DATABASE LINK mylink CONNECT TO ‘SYSTEM’ IDENTIFIED BY ‘password’ USING 'ORADB';

Create one to Oracle Database connection of database , Directly specify the connection string information ：

CREATE PUBLIC DATABASE LINK mylink CONNECT TO ‘SYSTEM’ IDENTIFIED BY ‘password’ USING
(DriverName=’Oracle ODBC Driver’, Host=192.168.0.1, Port=1521, Dbname=TEST, DbType=Oracle);

4.  modify Database Link object

ALTER DATABASE LINK To modify a dblink Database objects .

The first form changes dblink The owner of the , To modify the owner , You must have this dblink And also a direct or indirect member of the newly owned role .

The second form changes dblink The name of , Only dblink The owner or super user can rename a dblink.

Grammar format ：

ALTER [PUBLIC] DATABASE LINK dblink_name OWNER TO new_owner;
ALTER [PUBLIC] DATABASE LINK dblink_name RENAME TO new_name;

Parameter description ：

dblink_name

An existing dblink The name of （ It can be pattern Limited ）.

new_owner

The dblink The user name of the new owner of .

dblink_name

The dblink New name for .

annotation ：

ALTER DATABASE LINK The syntax is similar to ALTER VIEW.

give an example ：

hold DATABASE LINKkdb_dblink_regress.link_d The owner is modified to kdb_dblink_user:

ALTER DATABASE LINK kdb_dblink_regress.link_d OWNER TO kdb_dblink_user;

Compatibility ：

ALTER DATABASE LINK The statement is a KingbaseES Expand .

5.  Delete Database Link object

DROP DATABASE LINK Remove a dblink. To execute this command, you must have this dblink Authority .

Grammar format ：

DROP DATABASE LINK dblink_name

Parameter description ：

dblink_name

Delete the name dblink_name Of DATABASE LINK

annotation ：

DBLink Can be owned and DBA Delete , Delete DBLink After object , All objects in the user session will be cleared .

If DBLink Being used by other users , Cannot delete this DBLink.

Temporary does not support PUBLIC keyword .

Compatibility ：

The order is KingbaseES An extension of , compatible Oracle.

6. Database Link Permission control

For private DBLink, Its creator and DBA Have this DBLink Object to access . except DBLink Database users other than the creator can use PUBLIC Established by DBLink Visit .

The access rights of local users to remote database objects are controlled by the user authentication mechanism of the remote database system . adopt DBLink The local user who connects to the remote database will get the query permission owned by the user of the remote database .

7. DBLink Related views

DBLink The views provided to users are as follows ：

name	explain
ALL_DB_LINKS	Describe all database connections that the current user can access
USER_DB_LINKS	Describe all database connections owned by the current user
DBA_DB_LINKS	describe DBA All database connections that can be accessed

8. DATABASE LINK DML

DATABASE LINK — dblink Of DML operation

Grammar format ：

INSERT

[ WITH [ RECURSIVE ] with_query [, ...] ]
INSERT INTO [email protected] [ AS alias ] [ ( column_name [, ...] ) ]
    [ OVERRIDING { SYSTEM | USER} VALUE ]
    { VALUES ( { expression } [, ...] ) [, ...] | query }
    [ RETURNING * | output_expression [ [ AS ] output_name ] [, ...] ]

UPDATE

[ WITH [ RECURSIVE ] with_query [, ...] ]
UPDATE [email protected] [ * ] [ [ AS ] alias ]
    SET { column_name = { expression } |
        ( column_name [, ...] ) = [ ROW ] ( { expression } [, ...] ) |
        ( column_name [, ...] ) = ( sub-SELECT )
        } [, ...]
    [ FROM from_list ]
    [ WHERE condition ]
    [ RETURNING * | output_expression [ [ AS ] output_name ]  [, ...] ]

DELETE

[ WITH [ RECURSIVE ] with_query [, ...] ]
DELETE FROM [email protected] [ * ] [ [ AS ] alias ]
    [ WHERE condition ]
    [ RETURNING * | output_expression [ [ AS ] output_name ] [, ...] ]

describe ：

Use dblink Insert the remote table , to update , Delete .

Parameters ：

dblink

dblink The name of , Use CREATE DATABASE LINK Created dblink Name of the database connection .

with_query

WITH Clause allows you to specify one or more subqueries , stay INSERT UPDATE DELETE  The name of the subquery can be used to refer to them in the query See  WITH Inquire about （ Common table expressions ）  and  SELECT .

table_name

The name of an existing table （ Can be defined by patterns ）.

alias

Alternate name of the target table name . When an alias is provided , It will completely hide the actual name of the table .

column_name

be known as table_name  A column in the table of The name of , You can use the table name ( Can be qualified by the schema name ) Make a limit . If necessary, , Column names can also use a subdomain ( Subdomain name and table name are required 、 The schema name is qualified with parentheses ) Or array subscript qualification .

expression

The expression or value to be assigned to the corresponding column .

output_expression

After each row is inserted or updated or deleted by INSERT or UPUDATE or  DELETE The command calculates and returns expression . The expression can use ``table_name``  Any column in the specified table . It's written in * You can return all columns of inserted or updated or deleted rows .

output_name

The name to be used for the returned column .

condition

One can return boolean The expression of the value . Only let this expression return  true The row of will be updated , Delete .

Usage restriction ：

INSERT

Remote table insert operation is not supported ON CONFLICT Clause

The value of the specified generated column is not supported

about RETURNING, I won't support it RETURNING ctid Wait for the system column

The default value of column is not supported at present , For remote tables with default values , It's going on insert When , If you explicitly use, for example insert into {tablename} default value perhaps insert into {tablename} values(default) Will report a mistake dblink not support insert with default value; If insert into When , If you do not specify all the columns, the above error will be reported, such as when the table tab There are two columns a, b When , Then below sql All will report wrong.

1. insert into [email protected] values(1)

2. insert into [email protected](a) values(1)

3. insert into [email protected] select a from tab

4. insert into [email protected](a) select a from tab

UPDATE

When the remote table is an inherited table , Only updating parent table data is not supported , Appoint ONLY Keyword times error

about RETURNING, I won't support it RETURNING ctid Wait for the system column

at present update Default values for columns are not supported , It's going on update When , If you explicitly specify the default value , such as update {tablenaem} set {columname}=default, Will report a mistake dblink not support insert with default value

Currently not supported  WHERE CURRENT OF cursor_name  Usage of

at present update about   Partition table   and   Inheritance table   Your behavior is uncertain , So it's forbidden to use dblink Update remote   Partition table   perhaps   Inheritance table   Data in

DELETE

When the remote table is an inherited table , Deleting only parent table data is not supported , Appoint ONLY Keyword times error

about RETURNING, I won't support it RETURNING ctid Wait for the system column

at present delete about   Partition table   and   Inheritance table   Your behavior is uncertain , So it's forbidden to use dblink Delete remote   Partition table   perhaps   Inheritance table   Data in

ORACLE compatible ：

When dblink The database connection created is connected to Oracle When , In addition to the above use restrictions , at present dblink Not yet UPDATE and DELETE

9. DATABASE LINK MERGE

DATABASE LINK — dblink Of MERGE operation

Grammar format ：

MERGE INTO [ schema. ] { [email protected] } [ [ AS ] target_table_alias ]
    USING { [ schema. ] { source_table } [ [ AS ] source_table_alias ]
    ON ( condition_expression )
[ merge_update_clause ]
[ merge_insert_clause ];

merge_update_clause:
WHEN MATCHED THEN
UPDATE SET column = { expr }[, column = { expr } ]...
[ where_clause ]
[ delete_clause ]

delete_clause:
[DELETE where_clause]

merge_insert_clause:
WHEN NOT MATCHED THEN
INSERT [ ( column [, column ]...) ]
VALUES ({ expr }[, { expr } ]...)
[ where_clause ]

where_clause:
WHERE condition

describe ：

Use dblink Enter the remote table MERGE operation .

Parameters ：

target_table

MERGE The name of the remote target table .

dblink

dblink The name of , Use CREATE DATABASE LINK Created dblink Name of the database connection .

source_table

MERGE The name of the source table . The source table can be a local table , Ordinary appearance , It can also be a dblink Remote table of , namely [email protected]

target_table_alias

MERGE Alias of the target table .

source_table_alias

MERGE Alias of source table .

expr

The expression or value to be assigned to the corresponding column .

condition_expression

Specify the join conditions between the target table and the source table . If the condition is true , And it specifies WHEN MATCHED THEN UPDATE Clause , Then update the tuple of the matched target table ; otherwise , If the condition is false and WHEN NOT MATCHED THEN INSERT Clause , Then insert the target table .

merge_update_clause

When the target table and the source table ON When the condition is true , Execute the clause , That is, update the target table data . This update operation will trigger the trigger on the target table . The updated column cannot be ON The column referenced in the condition , When updating, you can use WHERE The condition indicates the row to be updated , Conditions can contain columns of the source table , You can also include the columns of the target table , When specified WHERE When the condition is false , Do not update .

delete_clause

DELETE Clause deletes only the target table and the source table ON Condition is true 、 And it is the updated record that meets the deletion conditions ,DELETE Clause does not affect INSERT Line inserted by item . Delete condition is applied to the updated record , It can be related to the source table , It can also be related to the target table , Or both . If ON Condition is true , But it does not meet the update conditions , No updated data , that DELETE No data will be deleted .

merge_insert_clause

When the target table and the source table ON When the condition is false , Execute the statement . You can specify the insertion condition , When inserting WHERE Conditions can only refer to columns in the source table .VALUES Only the columns in the source table can be referenced later , Cannot contain columns of the target table .