startsWith() does not use the PK index

[alex-zaitsev]

Describe the bug

startsWith(a, 'a') does not use the index, though functionally identical like 'a%' does. This is confusing.

How to reproduce

create table test_index (a String) Engine = MergeTree PARTITION BY tuple() ORDER BY a;

INSERT INTO test_index (a) values ('a');

select count() from test_index where startsWith(a, 'abc') settings force_primary_key=1;

Received exception from server (version 19.4.3):
Code: 277. DB::Exception: Received from localhost:9000, ::1. DB::Exception: Primary key (a) is not used and setting 'force_primary_key' is set.. 

Similar query with LIKE:

select count() from test_index where a like 'abc%' settings force_primary_key=1;

Ok.

Expected behaviour

startsWith to use index if possible.

[alexey-milovidov]

How to implement:
KeyCondition.cpp, look at atom_map and its usage.