Needs Suggestion

SUBHAM ROY <subham.iem@gmail.com> 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

Andreas
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On 05/04/2011 08:39 AM, Andreas Kretschmer wrote:

SUBHAM ROY <subham.iem@gmail.com> 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)

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Guillaume
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