SELECT 1 FROM test.dbo.a_003
gets about 60,000 records per second
SELECT '1' FROM test.dbo.a_003
gets about 600 records per second.
The cause is that postgres describes the return column as "unknown" length 65534 in the 2nd case.
Since the value is a constant, it seems rather odd to make the length 65534 characters. Why not make it char(1) or some other appropriate and less costly data type? After all, it isn’t going to grow during the query.
