1. Data Model: PostgreSQL is based on a traditional relational data model, where data is stored in tables with predefined schemas and relationships are established using foreign keys. On the other hand, QuestDB utilizes a time-series data model, which makes it particularly suited for storing and analyzing time-series data efficiently. This data model is optimized for continuous updates and queries on large volumes of time-stamped data.

2. Performance: QuestDB leverages various performance optimizations, including columnar storage, parallel processing, and hardware acceleration, to provide ultra-fast query speeds, especially for time-series data. PostgreSQL, on the other hand, excels in handling complex queries involving multiple tables and offers a wide range of indexing techniques for efficient data retrieval.

3. Scalability: PostgreSQL is known for its scalability and can handle large datasets and high transactional loads with ease. It supports a distributed architecture and provides features like sharding and replication for horizontal scalability. QuestDB, on the other hand, is optimized for high-throughput ingestion and query rates, making it suitable for applications that require extremely fast processing of time-series data.

4. SQL Compatibility: PostgreSQL fully complies with the SQL standard, offering a wide range of SQL features and functionalities. It also provides support for advanced SQL concepts like window functions, common table expressions, and recursive queries. QuestDB, being a specific purpose-built time-series database, supports a subset of SQL functionality, focusing primarily on time-series data operations.

5. Data Integration: PostgreSQL supports a variety of data formats, including JSON, XML, and geospatial data types, allowing for seamless integration with different types of applications and data sources. QuestDB, on the other hand, focuses primarily on time-series data and provides features specifically tailored for handling time-series datasets efficiently.

6. Community and Ecosystem: PostgreSQL has a large and active community with a vast ecosystem of extensions, tools, and libraries built around it. It has been in development for several decades and is supported by a dedicated group of developers. QuestDB, being relatively new, has a smaller community but is rapidly growing with an active GitHub repository and community forum.

In Summary, PostgreSQL and QuestDB differ in their data models, performance characteristics, scalability options, SQL compatibility, data integration capabilities, and community support. These differences make each database suitable for specific use cases and environments.