monitoring-stats.html documentation

Robert Haas <robertmhaas@gmail.com> writes:

http://developer.postgresql.org/pgdocs/postgres/monitoring-stats.html
says: "Note: blocks_fetched minus blocks_hit gives the number of
kernel read() calls issued for the table, index, or database; but the
actual number of physical reads is usually lower due to kernel-level
buffering." This seems to imply that anything that increases
blocks_hit should also increase blocks_fetched, but that doesn't seem
to match the actual behavior.

rhaas=# select heap_blks_read, heap_blks_hit from pg_statio_user_tables;

It's talking about the underlying pg_stat_get_db_blocks_fetched()
function, not heap_blks_read which is just a view field defined as

pg_stat_get_blocks_fetched(C.oid) -
pg_stat_get_blocks_hit(C.oid) AS heap_blks_read,

Probably that sentence ought to spell out the full function name
instead of abbreviating.

regards, tom lane

On Sat, Apr 4, 2009 at 6:08 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:

Robert Haas <robertmhaas@gmail.com> writes:

http://developer.postgresql.org/pgdocs/postgres/monitoring-stats.html
says: "Note: blocks_fetched minus blocks_hit gives the number of
kernel read() calls issued for the table, index, or database; but the
actual number of physical reads is usually lower due to kernel-level
buffering."  This seems to imply that anything that increases
blocks_hit should also increase blocks_fetched, but that doesn't seem
to match the actual behavior.

rhaas=# select heap_blks_read, heap_blks_hit from pg_statio_user_tables;

It's talking about the underlying pg_stat_get_db_blocks_fetched()
function, not heap_blks_read which is just a view field defined as

           pg_stat_get_blocks_fetched(C.oid) -
                   pg_stat_get_blocks_hit(C.oid) AS heap_blks_read,

Probably that sentence ought to spell out the full function name
instead of abbreviating.

Oh, I see. I misread it, but I agree it could be written in a way
that would make it less likely to be misread. You could even add a
sentence explicitly mentioning that the views display the difference
of the two values.

...Robert

Tom Lane wrote:

Robert Haas <robertmhaas@gmail.com> writes:

http://developer.postgresql.org/pgdocs/postgres/monitoring-stats.html
says: "Note: blocks_fetched minus blocks_hit gives the number of
kernel read() calls issued for the table, index, or database; but the
actual number of physical reads is usually lower due to kernel-level
buffering." This seems to imply that anything that increases
blocks_hit should also increase blocks_fetched, but that doesn't seem
to match the actual behavior.

rhaas=# select heap_blks_read, heap_blks_hit from pg_statio_user_tables;

It's talking about the underlying pg_stat_get_db_blocks_fetched()
function, not heap_blks_read which is just a view field defined as

pg_stat_get_blocks_fetched(C.oid) -
pg_stat_get_blocks_hit(C.oid) AS heap_blks_read,

Probably that sentence ought to spell out the full function name
instead of abbreviating.

Done.

--
Bruce Momjian <bruce@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ If your life is a hard drive, Christ can be your backup. +

Robert Haas wrote:

On Sat, Apr 4, 2009 at 6:08 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:

Robert Haas <robertmhaas@gmail.com> writes:

http://developer.postgresql.org/pgdocs/postgres/monitoring-stats.html
says: "Note: blocks_fetched minus blocks_hit gives the number of
kernel read() calls issued for the table, index, or database; but the
actual number of physical reads is usually lower due to kernel-level
buffering." ?This seems to imply that anything that increases
blocks_hit should also increase blocks_fetched, but that doesn't seem
to match the actual behavior.

rhaas=# select heap_blks_read, heap_blks_hit from pg_statio_user_tables;

It's talking about the underlying pg_stat_get_db_blocks_fetched()
function, not heap_blks_read which is just a view field defined as

? ? ? ? ? ?pg_stat_get_blocks_fetched(C.oid) -
? ? ? ? ? ? ? ? ? ?pg_stat_get_blocks_hit(C.oid) AS heap_blks_read,

Probably that sentence ought to spell out the full function name
instead of abbreviating.

Oh, I see. I misread it, but I agree it could be written in a way
that would make it less likely to be misread. You could even add a
sentence explicitly mentioning that the views display the difference
of the two values.

Done with attached patch; good idea.

--
Bruce Momjian <bruce@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ If your life is a hard drive, Christ can be your backup. +

On Thu, Apr 9, 2009 at 11:14 PM, Bruce Momjian <bruce@momjian.us> wrote:

Done with attached patch;  good idea.

Bruce,

You are a documentation-tuning machine... thanks.

...Robert