作者
digoal
日期
2020-11-15
标签
PostgreSQL , sql server , fdw , odbc , ogr , gdal , tds_fdw
背景
在PostgreSQL中通过外部表访问SQL Server的数据, 方法较多:
1、odbc_fdw
2、tds_fdw
3、ogr_fdw
4、multicorn sql_alchemy
Foreign Data Wrappers
Multicorn is bundled with a small set of Foreign Data Wrappers, which you can use or customize for your needs.
SQLAlchemy Foreign Data Wrapper
Class: multicorn.sqlalchemyfdw.SqlAlchemyFdw
Source code: multicorn/sqlalchemyfdw.py
Purpose
This fdw can be used to access data stored in a remote RDBMS. Through the use of sqlalchemy, many different rdbms engines are supported.
Dependencies
You will need the sqlalchemy library, as well as a suitable dbapi driver for the remote database.
You can find a list of supported RDBMs, and their associated dbapi drivers and connection strings in the sqlalchemy dialects documentation.
Required options
db_url (string)
An sqlalchemy connection string. Examples:
mysql: mysql://<user>:<password>@<host>/<dbname>
mssql: mssql://<user>:<password>@<dsname>
See the sqlalchemy dialects documentation. for documentation.
tablename (string)
The table name in the remote RDBMS.
Allowed options
primary_key (string)
Identifies a column which is a primary key in the remote RDBMS. This options is required for INSERT, UPDATE and DELETE operations.
When defining the table, the local column names will be used to retrieve the remote column data. Moreover, the local column types will be used to interpret the results in the remote table. Sqlalchemy being aware of the differences between database implementations, it will convert each value from the remote database to python using the converter inferred from the column type, and convert it back to a postgresql suitable form.
What does it do to reduce the amount of fetched data?
- quals are pushed to the remote database whenever possible. This include simple operators:
- equality, inequality (=, <>, >, <, <=, >=)
- like, ilike and their negations
- IN clauses with scalars, = ANY (array)
- NOT IN clauses, != ALL (array)
- the set of needed columns is pushed to the remote_side, and only those columns will be fetched.
Usage example
For a connection to a remote mysql database (you’ll need a mysql dbapi driver, such as pymysql):
```
CREATE SERVER alchemy_srv foreign data wrapper multicorn options (
wrapper 'multicorn.sqlalchemyfdw.SqlAlchemyFdw'
);
create foreign table mysql_table (
column1 integer,
column2 varchar
) server alchemy_srv options (
tablename 'table',
db_url 'mysql://myuser:mypassword@myhost/mydb'
);
```
参考
https://github.com/CartoDB/odbc_fdw
http://pgxn.org/dist/tds_fdw/
http://www.postgresonline.com/journal/archives/355-Connecting-to-SQL-Server-from-Linux-using-FDWs.html
http://www.postgresonline.com/article_pfriendly/355.html
https://programmer.help/blogs/postgresql-connects-to-sql-server-using-odbc_fdw.html
https://wiki.postgresql.org/wiki/Foreign_data_wrappers
/etc/odbc.ini
/etc/odbcinst.ini
https://pypi.org/project/pyodbc/
https://docs.sqlalchemy.org/en/14/dialects/mssql.html#dsn-connections
http://www.sqlalchemy.org/
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