On Fri, Jan 27, 2012 at 12:05 PM, Heikki Linnakangas <heikki.linnakangas@xxxxxxxxxxxxxxxx> wrote: > On 27.01.2012 19:43, Jon Nelson wrote: >> >> Let's say I have a 7GB table with 3-4 indices for a total of 10-12GB. >> Furthermore, let's say I have a machine with sufficient memory for me >> to set the work_mem and maintenance_work_mem to 20GB (just for this >> session). >> When I issue a CLUSTER using one of the indices, I see PostgreSQL (by >> way of strace) performing an index scan which amounts to large >> quantities of random I/O. >> In my case, that means it takes a very, very long time. PostgreSQL is >> largely at defaults, except for a 2GB shared_buffers and a few >> unrelated changes. The system itself has 32GB of physical RAM and has >> plenty free. >> Why didn't PostgreSQL just read the table into memory (and the >> interesting index) as a sequential scan, sort, and then write it out? >> It seems like there would be more than enough memory for that. The >> sequential I/O rate on this machine is 50-100x the random I/O rate. >> >> I'm using 8.4.10 (with the 'inet' de-toasting patch) on Scientific Linux >> 6.1. > > > The suppport for doing a seqscan+sort in CLUSTER was introduced in version > 9.1. Before that, CLUSTER always did an indexscan. See release notes: > http://www.postgresql.org/docs/9.1/static/release-9-1.html#AEN107416 That's what I get for digging through the source (git) but working with 8.4.10, on a Friday, at the end of a long week. Thanks for pointing that out to somebody that should have known better. -- Jon -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
