[wnoise]

All of the array types are basically the same. The docs actually do mention this, but only in passing as a current limitation.

https://www.postgresql.org/docs/13/arrays.html

> The current implementation does not enforce the declared number of dimensions either. Arrays of a particular element type are all considered to be of the same type, regardless of size or number of dimensions. So, declaring the array size or number of dimensions in CREATE TABLE is simply documentation; it does not affect run-time behavior.

Similar text extends at least all the way back to the documentation for 7.1.

[zeroimpl]

The other key to understanding this is that multidimensional arrays in Postgres are not simply arrays of arrays like typical programming languages. That’s why you can’t do things like array_agg on an array column.

[eyelidlessness]

Thanks! I don't know why I wasn't able to find it. Still, I would never have expected any of the behavior I described.

[sk5t]

I always internalized postgres's array behavior based on the examples, which include multidimensional arrays on text[] columns. Also, common functions like array_length require an index argument--"which array within the possibly multiple arrays do you want the length of" etc.