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 the Postgres optimizer uses:
SELECT (CASE WHEN reltuples > 0 THEN pg_relation_size('mytable')*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.