JDBC Foreign Data Wrapper for PostgreSQL

This is a foreign data wrapper (FDW) to connect PostgreSQL to any Java DataBase Connectivity (JDBC) data source.

This jdbc_fdw is based on JDBC_FDW and jdbc2_fdw.

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Contents

1. Features
2. Supported platforms
3. Installation
4. Usage
5. Functions
6. Identifier case handling
7. Generated columns
8. Character set handling
9. Examples
10. Limitations
11. Contributing
12. Useful links
13. License

Features

Common features

Write-able FDW

The existing JDBC FDWs are only read-only, this version provides the write capability. The user can now issue an insert, update, and delete statement for the foreign tables using the jdbc_fdw.

Arbitrary SQL query execution via function

Support execute the whole sql query and get results from the DB behind jdbc connection. This function returns a set of records.

Syntax:

jdbc_exec(text connname, text sql);

Example:
To get a set of record, use the below sql query:

SELECT jdbc_exec(jdbc_svr, 'SELECT * FROM tbl');
              jdbc_exec                 
----------------------------------------
 (1,abc,"Fri Dec 31 16:00:00 1999 PST")
 (2,def,"Fri Dec 31 16:00:00 1999 PST")

To get a set of record with separate data for each column, use the below sql query:

SELECT * FROM jdbc_exec(jdbc_svr, 'SELECT * FROM tbl') as t(id int, c1 text, c2 timestamptz);
 id |  c1 |              c2              
----+-----+------------------------------
  1 | abc | Fri Dec 31 16:00:00 1999 PST
  2 | def | Fri Dec 31 16:00:00 1999 PST

Pushdowning

WHERE clause push-down

The jdbc_fdw will push-down the foreign table where clause to the foreign server. The where condition on the foreign table will be executed on the foreign server, hence there will be fewer rows to bring across to PostgreSQL. This is a performance feature.

Column push-down

The existing JDBC FDWs are fetching all the columns from the target foreign table. The latest version does the column push-down and only brings back the columns that are part of the select target list. This is a performance feature.

Aggregate function push-down

List of aggregate functions push-down:

sum, avg, stddev, stddev_pop, stddev_samp, var_pop, var_samp, variance, max, min, count.

Notes about features

Maximum digits storing float value of MySQL

Maximum digits storing float value of MySQL is 6 digits. The stored value may not be the same as the value inserted.

Variance function

Variance function: For MySQL, variance function is alias for var_pop(). For PostgreSQL/GridDB, variance function is alias for var_samp(). Due to the different meaning of variance function between MySQL and PostgreSQL/GridDB, the result will be different.

Concatenation Operator

The || operator as a concatenation operator is standard SQL, however in MySQL, it represents the OR operator (logical operator). If the PIPES_AS_CONCAT SQL mode is enabled, || signifies the SQL-standard string concatenation operator (like CONCAT()). User needs to enable PIPES_AS_CONCAT mode in MySQL for concatenation.

Timestamp range

The MySQL timestamp range is not the same as the PostgreSQL timestamp range, so be careful if using this type. In MySQL, TIMESTAMP has a range of '1970-01-01 00:00:01' UTC to '2038-01-19 03:14:07' UTC.

JDBC connection

jdbc_fdw runs on the Postgres server and uses its IP address to make connections, which can make many trusted requests to the server's network. For example, when trusted local authentication is enabled in the PostgreSQL server, jdbc_fdw can connect to the loopback server (127.0.0.1) with any username (including the root account) without require password. So be careful when granting permissions of FOREIGN SERVER and USER MAPPING to non-supperuser.

Reference

Write-able FDW

The user can issue an update and delete statement for the foreign table, which has set the primary key option.

Supported platforms

jdbc_fdw was developed on Linux, and should run on any reasonably POSIX-compliant system.

jdbc_fdw is designed to be compatible with PostgreSQL 13 ~ 16. Java 5 or later is required (Confirmed version is Java OpenJDK 1.8.0).

Installation

Source installation

Prerequisites:

• JDBC driver(.jar file) of the database is needed. For example, jar file can be found from below: PostgreSQL GridDB

1. To build jdbc_fdw, you need to symbolic link the jvm library to your path.

sudo ln -s /usr/lib/jvm/[java version]/jre/lib/amd64/server/libjvm.so /usr/lib64/libjvm.so

2. Build

   make clean
   make install

   You may have to change to root/installation privileges before executing 'make install'

Usage

CREATE SERVER options

jdbc_fdw accepts the following options via the CREATE SERVER command:

• drivername as string

  The name of the JDBC driver. Note that the driver name has to be specified for jdbc_fdw to work. It can be found in JDBC driver documentation.

  • PostgreSQL drivername 'org.postgresql.Driver'
  • GridDB drivername 'com.toshiba.mwcloud.gs.sql.Driver'

• url as string

  The JDBC URL that shall be used to connect to the foreign database. Note that URL has to be specified for jdbc_fdw to work. It can be found in JDBC driver documentation.

• querytimeout as integer

  The number of seconds that an SQL statement may execute before timing out. The option can be used for terminating hung queries.

• jarfile as string

  The path and name(e.g. folder1/folder2/abc.jar) of the JAR file of the JDBC driver to be used of the foreign database. Note that the path must be absolute path.

  /[path]/[jarfilename].jar

• maxheapsize as integer

  The value of the option shall be set to the maximum heap size of the JVM which is being used in jdbc fdw. It can be set from 1 Mb onwards. This option is used for setting the maximum heap size of the JVM manually.

CREATE USER MAPPING options

jdbc_fdw accepts the following options via the CREATE USER MAPPING command:

• username

  The JDBC username to connect as.

• password

  The JDBC user's password.

CREATE FOREIGN TABLE options

jdbc_fdw accepts the no table-level options via the CREATE FOREIGN TABLE command.

The following column-level options are available:

• key as boolean

  The primary key options can be set while creating a JDBC foreign table object with OPTIONS(key 'true')

  CREATE FOREIGN TABLE [table name]([column name] [column type] OPTIONS(key 'true')) SERVER [server name];

  Note that while PostgreSQL allows a foreign table to be defined without any columns, jdbc_fdw can raise an error as soon as any operations are carried out on it.

IMPORT FOREIGN SCHEMA options

jdbc_fdw supports IMPORT FOREIGN SCHEMA (when running with PostgreSQL 9.5 or later) and accepts the following custom options:

• recreate as boolean

  If 'true', table schema will be updated. After schema is imported, we can access tables.

TRUNCATE support

jdbc_fdw yet don't implements the foreign data wrapper TRUNCATE API, available from PostgreSQL 14.

Functions

Functions from this FDW in PostgreSQL catalog are yet not described.

Identifier case handling

PostgreSQL folds identifiers to lower case by default, JDBC data source can have different behaviour. Rules and problems yet not tested and described.

Generated columns

Behaviour within generated columns yet not tested and described.

jdbc_fdw potentially can provide support for PostgreSQL's generated columns (PostgreSQL 12+).

Note that while jdbc_fdw will insert or update the generated column value in JDBC, there is nothing to stop the value being modified within JDBC, and hence no guarantee that in subsequent SELECT operations the column will still contain the expected generated value. This limitation also applies to postgres_fdw.

For more details on generated columns see:

• Generated Columns
• CREATE FOREIGN TABLE

Character set handling

Yet not described. JDBC is UTF-8 by default.

Examples

Install the extension:

CREATE EXTENSION jdbc_fdw;

Create a foreign server with appropriate configuration:

CREATE SERVER [server_name] FOREIGN DATA WRAPPER jdbc_fdw
    OPTIONS(
        drivername '[JDBC drivername]',
        url '[JDBC url]',
        querytimeout '[querytimeout]',
        jarfile '[the path of jarfile]',
        maxheapsize '[maxheapsize]'
    );

Create an appropriate user mapping:

CREATE USER MAPPING FOR CURRENT_USER SERVER [server name] 
    OPTIONS(username '[username]',password '[password]');

Create a foreign table referencing the JDBC table fdw_test:

CREATE FOREIGN TABLE [table name] (id int) SERVER [server name]);

Query the foreign table.

SELECT * FROM [table name];

Import a JDBC schema:

IMPORT FOREIGN SCHEMA public
  FROM SERVER [server name]
  INTO public
  OPTIONS (recreate 'true');

Limitations

Unsupported clause

The following clasues are not support in jdbc_fdw: RETURNING, GROUPBY, ORDER BY clauses, casting type, transaction control

Array Type

Currently, jdbc_fdw doesn't support array type.

Floating-point value comparison

Floating-point numbers are approximate and not stored as exact values. A floating-point value as written in an SQL statement may not be the same as the value represented internally.

For example:

SELECT float4.f1 FROM FLOAT4_TBL tbl06 WHERE float4.f1 <> '1004.3';
     f1
-------------
           0
      1004.3
      -34.84
 1.23457e+20
 1.23457e-20
(5 rows)

In order to get correct result, can decide on an acceptable tolerance for differences between the numbers and then do the comparison against the tolerance value to that can get the correct result.

SELECT float4.f1 FROM tbl06 float4 WHERE float4.f1 <> '1004.3' GROUP BY float4.id, float4.f1 HAVING abs(f1 - 1004.3) > 0.001 ORDER BY float4.id;
     f1
-------------
           0
      -34.84
 1.23457e+20
 1.23457e-20
(4 rows)

IMPORT FOREIGN SCHEMA

Currently, IMPORT FOREIGN SCHEMA works only with GridDB.

Contributing

Opening issues and pull requests on GitHub are welcome.

Useful links

Source code

• JDBC_FDW

• jdbc2_fdw

  Reference FDW realisation, postgres_fdw

• https://git.postgresql.org/gitweb/?p=postgresql.git;a=tree;f=contrib/postgres_fdw;hb=HEAD

General FDW Documentation

• https://www.postgresql.org/docs/current/ddl-foreign-data.html
• https://www.postgresql.org/docs/current/sql-createforeigndatawrapper.html
• https://www.postgresql.org/docs/current/sql-createforeigntable.html
• https://www.postgresql.org/docs/current/sql-importforeignschema.html
• https://www.postgresql.org/docs/current/fdwhandler.html
• https://www.postgresql.org/docs/current/postgres-fdw.html

Other FDWs

• https://wiki.postgresql.org/wiki/Fdw
• https://pgxn.org/tag/fdw/

License

Copyright (c) 2021, TOSHIBA CORPORATION Copyright (c) 2012 - 2016, Atri Sharma atri.jiit@gmail.com

Permission to use, copy, modify, and distribute this software and its documentation for any purpose, without fee, and without a written agreement is hereby granted, provided that the above copyright notice and this paragraph and the following two paragraphs appear in all copies.

See the LICENSE file for full details.