Why does PostgreSQL perform sequential scan on indexed column?

If the SELECT returns more than approximately 5-10% of all rows in the table, a sequential scan is much faster than an index scan.

This is because an index scan requires several IO operations for each row (look up the row in the index, then retrieve the row from the heap). Whereas a sequential scan only requires a single IO for each row - or even less because a block (page) on the disk contains more than one row, so more than one row can be fetched with a single IO operation.

Btw: this is true for other DBMS as well - some optimizations as "index only scans" taken aside (but for a SELECT * it's highly unlikely such a DBMS would go for an "index only scan")

Did you ANALYZE the table/database? And what about the statistics? When there are many records where year > 2009, a sequential scan might be faster than an index scan.

@a_horse_with_no_name explained it quite well. Also if you really want to use an index scan, you should generally use bounded ranges in where clause. eg - year > 2019 and year < 2020.

A lot of the times statistics are not updated on a table and it may not be possible to do so due to constraints. In this case, the optimizer will not know how many rows it should take in year > 2019. Thus it selects a sequential scan in lieu of full knowledge. Bounded partitions will solve the problem most of the time.