We don't always need an accurate count of rows, especially on a large table that may take a long time to execute. Administrators often need to estimate how big a table is so that they can estimate how long other operations may take.
We can get a quick estimate of the number of rows in a table using roughly the same calculation that Postgres optimizer uses:
SELECT (CASE WHEN reltuples > 0 THEN pg_relation_size(oid)*reltuples/(8192*relpages) ELSE 0 END)::bigint AS estimated_row_count FROM pg_class WHERE oid = 'mytable'::regclass;
This gives us the following output:
estimated_count --------------------- 293 (1 row)
It returns a row count very quickly, no matter how large the table that we are examining is. You may want to create a SQL function for the preceding calculation, so you won't need to retype the SQL code every now and then.
The following function estimates the total number of rows using a mathematical procedure...