A Cross-Database Journey: Accessing Objects in Other PostgreSQL Databases


PostgreSQL is a widely used open-source object-relational database management system (ORDBMS) that is designed to handle various workloads ranging from small to large-scale web applications. One of the advantages of PostgreSQL is its ability to support multiple databases within a single server instance. A PostgreSQL database consists of a collection of tables, views, indexes, and other objects that are organized into schemas.

In modern application development, it is common for developers to work with multiple databases to achieve complex business requirements or scaling needs. In such scenarios, accessing objects in other databases becomes crucial but often poses challenges due to differences in data structure, security policies and access privileges.

In this article, we will explore the cross-database journey and the different techniques for accessing objects in other PostgreSQL databases. We will also discuss the security considerations when working with multiple databases and highlight best practices for securing cross-database access.

Explanation of PostgreSQL Databases

A PostgreSQL database is a collection of related data organized into tables that can be accessed by authorized users via SQL queries or programming interfaces. Each database can have one or more schemas that contain tables and other relational objects such as views, indexes, functions, triggers and stored procedures. PostgreSQL supports different types of databases including single-user databases which are created when a user runs the “createdb” command or multi-user databases which allow multiple users to access the same set of data without interfering with each other’s operations.

One advantage of using multiple-databases within a single Postgres instance is that it provides better isolation between different applications or modules within an application. Additionally, it allows developers to experiment with new features or changes without affecting production data.

Importance of Accessing Objects in Other Databases

In modern application development, there are several scenarios where accessing objects in other databases becomes essential. For example, a web application that deals with user authentication might need to access user data stored in a separate database. Or an e-commerce application that deals with orders and payments may require accessing product information from another database.

Accessing objects in other databases within the same PostgreSQL instance can help reduce complexity and improve performance as it eliminates the need for data duplication or synchronization. Also, it enables developers to write more efficient queries that join tables from multiple databases rather than using complex sub-queries or manual data processing.

Overview of the Cross-Database Journey

Cross-database access is the process of querying or manipulating objects that reside in another database within the same PostgreSQL instance. The cross-database journey typically involves understanding the structure and relationships between different databases, selecting an appropriate technique for accessing remote objects, configuring security settings and testing connectivity.

In general, there are two types of techniques for cross-database access: Foreign Data Wrappers (FDWs), which enable a PostgreSQL server to connect to external data sources such as other relational databases or web services and Remote Procedure Calls (RPCs), which allow procedures to be executed on a remote database via SQL commands. This article will explore these two techniques in detail along with their benefits, limitations and use cases.

Understanding PostgreSQL Databases

PostgreSQL is a powerful and open-source relational database management system that provides a flexible and robust environment for developing database applications. A PostgreSQL database is essentially a collection of data organized into tables, where each table has its own set of columns and rows that store specific information.

There are two main types of PostgreSQL databases: single-database systems and multiple-database systems. In a single-database system, there is only one database where all the data is stored.

This type of system is suitable for small projects or personal use as it provides simplicity in terms of managing the data structure. In contrast, multiple-database systems allow users to create separate databases that can be managed independently, each with its own set of tables, schemas, permissions, and security settings.

Features and Benefits of Using PostgreSQL Databases

PostgreSQL has many features that make it an ideal choice for developing robust database applications. These features include support for different data types such as arrays, JSON documents, hstore key-value pairs; transactional integrity; multi-version concurrency control; extensibility through custom functions and procedural languages such as PL/pgSQL; scalability using specialized partitioning techniques such as range or hash partitioning. Another great benefit of using PostgreSQL databases is the flexibility they provide when it comes to scaling out your application.

You can easily add more nodes or replicas to your cluster without downtime by setting up asynchronous replication or logical replication between nodes. Furthermore, when you use PostgreSQL databases in combination with advanced indexing techniques like B-tree indexes or GiST (Generalized Search Tree) indexes you can achieve high-performance queries even on large datasets.

Differences between Single and Multiple Database Systems

Single-database systems are best suited for small-scale projects where there isn’t much need for complex query processing or advanced scalability functionality. They provide simplicity but lack flexibility when it comes to managing different types of data or scaling out the database.

In contrast, multiple-database systems provide a more complex but powerful toolset that enables users to create separate databases for different use cases. This functionality enables users to control the security settings and permissions of each database separately, ensuring maximum flexibility and control over data management.

Overall, PostgreSQL is an excellent choice for developing robust database applications that require scalability, flexibility and advanced query processing capabilities. With its rich feature set and flexible architecture, it can help you build powerful applications that meet your business needs while being cost-effective in terms of licensing and support costs.

Cross-Database Accessing Techniques

When working with PostgreSQL, there are several techniques that can be used to access objects in other databases. Two popular methods are Foreign Data Wrappers (FDWs) and the dblink extension.

Foreign Data Wrappers (FDWs)

Foreign Data Wrappers (FDWs) are a type of PostgreSQL extension that provides a way to access external data sources such as other databases or web APIs. FDWs act as a bridge between PostgreSQL and external data sources, allowing you to query and manipulate data in these sources from within your PostgreSQL database.

To install an FDW in your PostgreSQL database, you must first download the appropriate package for your operating system and version of PostgreSQL. Once downloaded, you can use the CREATE EXTENSION command in psql or pgAdmin to install the FDW.

After installation, you can use SQL commands to create foreign tables that reference tables in other databases or data sources. There are several popular FDWs available for cross-database access in PostgresSQL.

Some examples include:

  • postgres_fdw: this fdw allows you to access tables in another postgresql database.
  • mysql_fdw: this fdw allows you to access tables in a mysql database.
  • couchbase_fdw: this fdw allows you to access couchbase nosql databases.

The dblink Extension

The dblink extension provides another method for accessing objects in other PostgreSQL databases. It allows you to execute queries against remote databases using SQL commands. To use the dblink extension, it must first be installed on both the local and remote servers.

Once installed, you can use the dblink function to execute queries on the remote server. The syntax for calling dblink is: SELECT * FROM dblink(‘dbname=remote_dbname host=remote_host user=remote_user password=remote_password’, ‘SELECT * FROM remote_table’) AS t(column1 datatype, column2 datatype);

This command will execute the SELECT statement on the remote server and return the results as a table in your local database. The dblink extension provides a simple and efficient method for accessing objects in other PostgreSQL databases. However, it should be noted that there can be security risks associated with using dblink, as it requires storing credentials for the remote database in your local database.

Cross-Database Querying Methods

Remote Procedure Calls (RPCs)

In PostgreSQL, Remote Procedure Calls (RPCs) are a popular cross-database querying method. RPCs enable remote execution of procedures and functions on another PostgreSQL database.

This means that users can easily access objects in other databases without physically moving or copying them to a different location. Instead, RPCs provide a way for applications to call procedures on remote databases while maintaining the results within the current database.

RPC-based querying has significant benefits over other methods that require data duplication or synchronization between databases. For example, with RPC-based querying, data consistency can be maintained across all databases because changes made in one database are immediately reflected in others as long as they share the same procedures or functions.

Definition and explanation of RPCs

A remote procedure call (RPC) enables an application to execute code on a remote machine or server without requiring local data storage or processing power. It provides an abstraction layer between the application code and the remote service, allowing developers to focus only on the desired functionality rather than low-level network communication details.

In PostgreSQL, an RPC is typically defined as a function or procedure that is executed by one database server but resides on another server machine. The result of this execution is then returned back to the calling application for further processing.

How to use RPCs for cross-database querying

To use RPCs in PostgreSQL, we must create a function that will execute a specific query and return results from another database server using language extensions such as PL/Perl. Once this function has been created, it can then be invoked from any client application within our environment by passing parameters such as connection details and query strings.

For security reasons, it is important to grant access privileges at both ends before executing any queries via RPCs. This includes granting read-only or read-write access to specific tables or schema objects in the target database and limiting the scope of the user’s privileges.

Examples of RPCs usage for cross-database querying

One example of an RPC-based application is a distributed inventory management system that requires access to multiple databases across different geographical locations. With RPCs, the application can execute queries on remote servers without creating duplicate copies of data.

This enables real-time updates to inventory levels, which ensures accurate stock information across all locations. Another example is a financial data analysis tool that pulls data from multiple databases.

By using RPC-based queries, traders can quickly and easily access real-time market data for different securities and generate visualizations to support their investment decisions. In both these examples, RPC-based querying is used to eliminate the need for repetitive data synchronization processes and provides a more efficient way of accessing objects in other PostgreSQL databases.

Cross-Database Security Considerations

Accessing objects in other databases can be a security risk if not properly secured. When accessing objects in other PostgreSQL databases, it is important to ensure that confidential data is not exposed to unauthorized parties.

Some of the security risks associated with accessing objects in other databases include:

  • Data Breaches: accessing data from unsecured or poorly secured databases can lead to data breaches and loss of confidential information.
  • Data Corruption: accessing or modifying data from multiple databases using untested or unstable cross-database techniques can corrupt the entire database system.
  • Data Loss: poorly written code or configuration errors in cross-database queries can result in permanent loss of data in one or more database systems.

To mitigate these risks, proper measures must be put in place to secure cross-database access.

Best practices for securing cross-database access

To ensure secure access to objects in other PostgreSQL databases, several best practices should be followed. These include:

  • User Permissions: Only users authorized to access specific objects should have permissions granted on those objects. It is also recommended that passwords are encrypted and stored securely on both the client and server sides.
  • Data Encryption: all sensitive data transmitted between different database systems should be encrypted using secure encryption algorithms such as ssl (secure sockets layer) protocol.
  • Auditing and Logging: Keeping track of all user activities and queries executed against the database system helps identify any suspicious activity or unauthorized access attempts. This ensures early detection of any potential security breach attempts, making recovery more straightforward and timely.
  • Data Segregation: Separating data from different departments or business units helps maintain data privacy and security.

It is recommended to create separate schemas or databases for each department, which can be accessed only by authorized users. This helps prevent accidental sharing of sensitive information between different departments.

  • Testing and Deployment: Before deploying any cross-database querying techniques, it is essential to perform rigorous testing on a development environment to detect and fix any anomalies. This ensures that the production environment is stable and secure, hence minimizing the risk of data corruption or loss.
  • Regular Security Auditing: Regular audits are essential in identifying potential vulnerabilities in the database system. It is recommended that external security auditors conduct periodic audits to identify any weaknesses in the database system’s security protocols.

By following these best practices, businesses can not only ensure secure access to objects in other PostgreSQL databases but also minimize the risk of losing valuable confidential information, ensuring a more secure database environment overall.


Accessing objects in other PostgreSQL databases can be highly beneficial for businesses using multiple database systems. The use of Foreign Data Wrappers (FDWs) and dblink extension make cross-database access simple and seamless.

FDWs provide a flexible and efficient way to access remote data sources while dblink extension is an easy-to-use method for querying remote databases. However, it is important to take security considerations seriously when accessing objects across different databases.

Proper authentication and encryption protocols should be implemented to ensure sensitive data is not compromised during the cross-database journey. Through this article, we have covered the basics of PostgreSQL databases, the differences between single and multiple database systems, as well as various techniques used to access objects in other databases.

We explored Foreign Data Wrappers (FDWs), dblink extension, Remote Procedure Calls (RPCs), and considered security measures that should be taken into account when implementing cross-database access. Overall, by understanding PostgreSQL databases and implementing proper cross-database accessing techniques with security measures in place, businesses can experience increased efficiency and productivity through better data management practices.

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