On Fri, Jan 27, 2012 at 7:34 PM, Jon Nelson <jnelson+pgsql@xxxxxxxxxxx> wrote: > 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. But if you're stuck on < 9.1 for a while, the workaround is to cluster the table yourself by using a select * ... order by pkey. For randomly distributed tables this is far faster for a first time cluster. After that, subsequent clusters won't have as much work to do and the older method for clustering should work ok. It's kinda funny to have a complaint against pgsql for NOT using a sequential scan. Most DBAs that come from other DBAs are upset when it doesn't use an index. -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
