Also keep in mind that you may have several PL/pgSQL functions in your call
stack, and you'll want to capture the local variables at each level.
Yes, the format of the string written to the log must support multi-level data strucutres, perhaps JSON would be a good choice.
Consider the potential need to avoid logging very-large variable values. The GUC could perhaps be a size limit (0 disables the feature entirely), not a boolean.
Yes, for very large values, it's okay if they are truncated or perhaps truncated+a simple MD5-hash of the value, so you can check if it is equal to the excepted value. You are usually not interested in these anyway, since what you are looking for are probably things like ID-numbers, usernames, etc.
