On 05/04/2011 08:39 AM, Andreas Kretschmer wrote: > SUBHAM ROY <subham.iem@xxxxxxxxx> wrote: > >> My output of explain (analyze,buffers) is something like this: >> >> shared hit=3796624 read=46038 >> >> So what is meant by read here? Does it indicates number of disk reads? >> >> Does shared hit takes into account only the hit in the pg_buffercache of the >> postgres or it also takes into account the linux buffers that postgres uses? > > shared hit: read from shared mem > shared read: read from disk into shared mem > To be more precise: shared hit number of blocks directly read from PostgreSQL shared memory read number of blocks PostgreSQL asked the OS to give him (then the OS may read the blocks from its own shared memory or may ask the disk subsystem to give him (and then the disk subsystem may read it from its own shared memory (SAN cache for example) or read it from the disk)) To put it more simply: shared hit: number of blocks directly read from PostgreSQL shared memory read: number of blocks read outside of PostgreSQL shared memory (meaning it may need disk read) -- Guillaume http://www.postgresql.fr http://dalibo.com -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
