Hello, I'm currently have to investigate some time to anyalyze how often the database has to read data from disk compared to the number of cache(shared buffer) accesses. I got the following key figures for an example table: a) pg_statio_user_indexes=> -[ RECORD 12 ]+---------------------------------- relid | 42535 ... idx_blks_read | 20504593 idx_blks_hit | 17756649 -[ RECORD 14 ]+---------------------------------- relid | 42535 ... idx_blks_read | 146942531 idx_blks_hit | 48752405641 Regarding "RECORD 12" I can't explain why the number of blks_read is higher than the number of blks_hit. Why should a page be read if we can't find a requested tuple there ? The numbers in "RECORD 14" seems O.K., we read some pages and we find several tuples we need in each page (for average). Is this assumption correct ? b) pg_statio_user_tables=> -[ RECORD 2 ]---+------------ relid | 42535 .. heap_blks_read | 1572252620 heap_blks_hit | 32724990601 idx_blks_read | 197453378 idx_blks_hit | 49240726062 toast_blks_read | 0 toast_blks_hit | 0 tidx_blks_read | 0 tidx_blks_hit | 0 heap_blks_read is the number of disk blocks read for that table (excluding index access), does heap_blks_hit mean the number of accesses to the cache for that data ? ...and is the number of heap_blks_read in heap_blks_hit included, or is this number the additional accesses, after reading the data from disk to buffer ? Let me try to explain my question with the numbers of the example table: number of disk reads = 1572252620 number of cache reads = 32724990601 OR (32724990601 - 1572252620) ??? any help appreciated.... many thanks in advance...GERD.. -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
