autoanalyze / autovacuum vs manually executed "vacuum analyze"

Hi everyone,

we created an index on a table to improve the performance for a SQL statement. After executing “vacuum analyze mytable”, the index is used. I know that there is also an autovacuum/autoanalyzer configured. I can see that autovacuum and autoanalyzer ran recently. But the index is still not used. I would expect that the index is used after the autovacuum/autoanalyze.

Questions:

1. Why would the index not be used after autovacuum/autoanalyze but only after manually running vacuum analyze?
2. Is there any difference between the autovacuum /autoanalyze and the manual exected “vacuum analyze”?

Details:

1) Get last analyze and last autoanalyzer dates:
select * from pg_stat_all_tables where relname ='mytable'

"last_analyze": "2024-05-07T15:26:01.363796+00:00",
"last_autoanalyze": "2024-06-09T20:52:32.411717+00:00",
"last_autovacuum": "2024-05-20T02:14:34.165689+00:00",
"last_vacuum": "2024-05-07T15:24:42.644449+00:00",

2) Explain analyze <SQL statement>: no index is used for the SQL statement

3) vacuum analyze mytable -- manually executed

4) Explain analyze <SQL statement>: index is now used ✓

Thanks in advance &
Best regards,
Manuel