I encountered a similar issue while attempting to migrate my MySQL database to PostgreSQL using pgloader, specifically the error:

MySQL 7.6 fell through ECASE expression at /build/pgloader-4.0.0~beta1/build/sources/libs/dbd-mysql/mysql.c:1638

After some investigation, I realized the problem stemmed from the way pgloader was built. I had cloned the repository from GitHub, assuming a standard build process would suffice. However, I overlooked a crucial step in the installation documentation.

The key to resolving this error, as highlighted in the official pgloader documentation (https://pgloader.readthedocs.io/en/latest/install.html#build-from-sources), is to explicitly run the $ make save command after cloning the repository.

This command is essential as it handles specific build configurations necessary for pgloader to interact correctly with different database versions. Simply running a generic make command might not include these crucial steps.

Therefore, the solution that worked for me was:

1. Clone the pgloader repository:

   Bash

   git clone https://github.com/dimitri/pgloader.git
   cd pgloader
   
   

2. Execute the $ make save command:

   Bash

   make save
   
   

By ensuring the $ make save command is executed, the necessary build artifacts are created, allowing pgloader to handle the nuances of the MySQL version and successfully proceed with the migration.

Hopefully, this clarifies the issue and helps others facing the same error during their MySQL to PostgreSQL migration with pgloader!