kelphet xiong <kelphet@xxxxxxxxx> wrote: > When I use postgres and issue a simple sequential scan for a > table inventory using query "select * from inventory;", I can see > from "top" that postmaster is using 100% CPU, which limits the > query execution time. My question is that, why CPU is the > bottleneck here and what is postmaster doing? Is there any way to > improve the performance? Thanks! > explain analyze select * from inventory; > > Seq Scan on inventory (cost=0.00..180937.00 rows=11745000 width=16) (actual time=0.005..1030.403 rows=11745000 loops=1) > Total runtime: 1750.889 ms So it is reading and returning 11.7 million rows in about 1 second, or about 88 nanoseconds (billionths of a second) per row. You can't be waiting for a hard drive for many of those reads, or it would take a lot longer, so the bottleneck is the CPU pushing the data around in RAM. I'm not sure why 100% CPU usage would surprise you. Are you wondering why the CPU works on the query straight through until it is done, rather than taking a break periodically and letting the unfinished work sit there? -- Kevin Grittner EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
