Counting is one of the easiest SQL statements, so it is also many people's first experience of a Postgres query.
From any interface the SQL command is, as follows:
SELECT count(*) FROM table;
which returns a single integer value as the result.
In psql this looks like the following:
postgres=# select count(*) from orders; count ─────── 345 (1 row)
count(*) function will scan every row in the table using a technique named a Sequential Scan. We access every data block in the table one after the other, reading the number of rows in each block. If the table is on disk, then this will cause a beneficial disk access pattern, and the statement will be fairly fast.
Some people think that the count ...