doc(mysql): document difference between Uuid and `uuid::fmt::Hyphen…

…ated`

sqlx-mysql/src/types/mod.rs

@@ -115,9 +115,33 @@
 //!
 //! | Rust type                             | MySQL/MariaDB type(s)                                |
 //! |---------------------------------------|------------------------------------------------------|
-//! | `uuid::Uuid`                          | BINARY(16), VARCHAR, CHAR, TEXT                      |
-//! | `uuid::fmt::Hyphenated`               | CHAR(36), UUID (MariaDB-only)                        |
-//! | `uuid::fmt::Simple`                   | CHAR(32)                                             |
+//! | `uuid::Uuid`                          | BINARY(16) (see note)                                |
+//! | `uuid::fmt::Hyphenated`               | CHAR(36), VARCHAR, TEXT, UUID (MariaDB-only)         |
+//! | `uuid::fmt::Simple`                   | CHAR(32), VARCHAR, TEXT                              |
+//! 
+//! #### Note: `Uuid` uses binary format
+//! 
+//! MySQL does not have a native datatype for UUIDs. 
+//! The `UUID()` function returns a 36-character `TEXT` value, 
+//! which encourages storing UUIDs as text.
+//! 
+//! MariaDB's `UUID` type stores and retrieves as text, though it has a better representation
+//! for index sorting (see [MariaDB manual: UUID data-type][mariadb-uuid] for details). 
+//! 
+//! As an opinionated library, SQLx chose to map `Uuid` to/from binary format by default
+//! (16 bytes, the raw value of a UUID; SQL type `BINARY(16)`). 
+//! This saves 20 bytes over the text format for each value.
+//! 
+//! The `impl Decode<MySql> for Uuid` does not support the text format, and will return an error.
+//! 
+//! If you want to use the text format compatible with the `UUID()` function,
+//! use [`uuid::fmt::Hyphenated`][::uuid::fmt::Hyphenated] in the place of `Uuid`.
+//! 
+//! The MySQL official blog has an article showing how to support both binary and text format UUIDs
+//! by storing the binary and adding a generated column for the text format, though this is rather
+//! verbose and fiddly: <https://dev.mysql.com/blog-archive/storing-uuid-values-in-mysql-tables/>
+//! 
+//! [mariadb-uuid]: https://mariadb.com/kb/en/uuid-data-type/
 //!
 //! ### [`json`](https://crates.io/crates/serde_json)
 //!

sqlx-mysql/src/types/uuid.rs

@@ -33,6 +33,15 @@ impl Decode<'_, MySql> for Uuid {
         // delegate to the &[u8] type to decode from MySQL
         let bytes = <&[u8] as Decode<MySql>>::decode(value)?;
 
+        if bytes.len() != 16 {
+            return Err(format!(
+                "Expected 16 bytes, got {}; `Uuid` uses binary format for MySQL/MariaDB. \
+                 For text-formatted UUIDs, use `uuid::fmt::Hyphenated` instead of `Uuid`.",
+                bytes.len(),
+            )
+            .into());
+        }
+
         // construct a Uuid from the returned bytes
         Uuid::from_slice(bytes).map_err(Into::into)
     }