First, I query a table in descending order.

select first 250 * from table_A A
where A.time > sometimestamp
order by A.ID desc

ID is the primary key. After creating a descending index on ID, the
query takes about 50ms.

I need to furtherly join table_A to a small table with only 5 records.
Now the query is revised as:

select first 250 * from table_A A
left join table_B B
on A.column = B.column
where A.time > sometimestamp
order by A.ID desc

Now it takes more than 1 minute :-(

Can anyone please point out what is missing here?

Best,
Sean