[Fixed]-Why does Django create an index on a unique field explicitly

The heart of the issue is this guarantee made in the Django documentation:

Note that when unique is True you don’t need to specify db_index, because unique implies the creation of an index.

So by Django’s contract, unique=True implies db_index=True, and db_index=True means that Django has to create the text_pattern_ops index to support all lookup types (see ticket 12234).

As for just using a single unique index, the PostgreSQL documentation says that that won’t cover all lookup types:

Note that you should also create an index with the default operator class if you want queries involving ordinary <, <=, >, or >= comparisons to use an index. Such queries cannot use the xxx_pattern_ops operator classes.

You can try adding both unique=True and db_index=False.