It's not unusual to find an application that does the following to determine how many rows there are in a table:
SELECT count(*) FROM t;
In some databases other than PostgreSQL, this executes very quickly, usually because that information is kept handy in an index or similar structure. Unfortunately, because PostgreSQL keeps its row visibility information in the row data pages, you cannot determine a true row count without looking at every row in the table, one at a time, to determine whether they are visible or not. That's a sequential scan of the full table, and it's pretty slow; it even turns out to be an effective way to benchmark sequential read speed on a table to count its rows this way!
If your statement is fairly selective ...
