Why are there unknown characters such as a left curly brace in the Postgres protocol?

The presence of seemingly "unknown" characters, such as a left curly brace {, in the Postgres protocol is typically due to the way Postgres handles data types, particularly when dealing with complex or composite types like arrays, JSON, or custom types.

Here's a breakdown of why these characters appear:

1. Data Type Representation:

- Postgres uses a text-based protocol for communication. When sending data, it often represents complex data structures as text strings. The curly braces {} are used to denote arrays. For example, an array of integers [1, 2, 3] might be represented as {1,2,3} in the protocol.

2. Composite Types:

- Postgres allows the creation of custom composite types, which are essentially records or structs. These types can also be represented using curly braces to enclose their fields. For instance, a composite type representing a point (x, y) might be represented as (10,20) or "(10,20)" in the protocol.

3. JSON and JSONB:

- When dealing with JSON or JSONB data types, the curly braces are part of the JSON syntax itself. Postgres will transmit JSON data as a string, which includes these braces. For example, a JSON object {"key": "value"} will be sent as is.

4. Escaping and Quoting:

- Sometimes, these characters might be escaped or quoted to ensure they are interpreted correctly. For example, if a string value contains a curly brace, it might be escaped to avoid confusion with array or composite type delimiters.

5. Protocol Details:

- The specific format of the data depends on the message type and the data type being transmitted. The Postgres protocol is well-defined, and these characters are part of that definition. The protocol uses a combination of text and binary formats, but the text representation is common for many data types.

6. Client-Side Interpretation:

- It's important to note that these characters are not "unknown" to the Postgres client or driver. The client is responsible for parsing the data according to the protocol and converting it into the appropriate data structures in the client's programming language.

In summary, the curly braces and other seemingly "unknown" characters are part of the Postgres protocol's way of representing complex data types as text strings. These characters are essential for the correct transmission and interpretation of data between the Postgres server and its clients. Understanding this representation is crucial when working with raw protocol data or debugging communication issues.

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