PostgreSQL - why does the index not work on 50000 records?

Question

PostgreSQL

Greetings, dear connoisseurs!!!
Such a situation: there is a table foo with columns id, active, field_id1, field_id2 and there is a composite index foo_field_id1_active with fields field_id1, acitve. Query for 10,000 records
EXPLAIN SELECT * FROM foo WHERE filed_id1=4444 AND active=1
Indicates that the index is in use, but at 50,000 records it is no longer used even after VACUUM ANALIZE foo;
About 1,000,000 records, I generally keep quiet.
And so the question is: which artist, the index is no longer used?

Answer 1

Postgresql considered that seq scan would be faster.
Why don't they use my indexes?

Answer 2

And so the question is: which artist, the index is no longer used?
Malevich