Eh, I checked, you are right. Something terribly wrong with hash index in PostgreSQL. But there are another hash index gin(jsonb_path_ops), may be correctly say gin+hash index. Looked like it is the best for this purpose.
"GIN, the most know non-default index type perhaps, has been actually around for ages (full-text search) and in short is perfect for indexing columns where there are lot of repeating values – think all kinds of statuses or good old Mr/Mrs/Miss. GIN only stores every unique column value only once as for the default B-tree you’ll have e.g. 1 millon leaf nodes with the integer “1” in it."
Does it mean that GIN is a very good choice for low cardinality columns.
Not necessary. There is other index which also don’t keep column value in an every leaf. Hash, for instance.
For smallish values (which low cardinality columns tend to be) the per-tuple overhead and the pointer itself is probably much larger than the value, so hash won't save you much if any space. The GIN index removes not just the value, but the per-tuple overhead. And also compresses the point list to further save space.
Here is a real-world example from one of my databases where each value is about 17 characters long, and is present about 20 times:
