psql “select id from my_table" | sort -u | wc -l
That will be a lot slower than just "select count(*) from
my_table". You are delivering data to the user program (psql) and
then shipping them to pipe and then processing the output with
"wc". Depending on the version, PostgreSQL has very reliable
parallelism and can do counting rather quickly. The speed of
"select count(*) from my_table" depends on the speed of I/O. Since
the table is big, it cannot be cached in the file system cache, so
all that you have at your disposal is the raw disk speed. For the
smaller machines, NVME is the king. For larger rigs, you should
consider something like Pure, XTremIO or NetApp SolidFire. People
frequently expect database to do miracles with under par hardware.
-- Mladen Gogala Database Consultant Tel: (347) 321-1217 https://dbwhisperer.wordpress.com
