Hello Craig:
Thank you very much!
-----邮件原件-----
发件人: Craig Ringer [mailto:ringerc@ringerc.id.au]
发送时间: 2012年7月27日 11:00
收件人: leoxu
抄送: pgsql-general@postgresql.org
主题: Re: 答复: [GENERAL] how to calculate or know seq_scan scan how many
blocks every time
On 07/27/2012 10:04 AM, leoxu wrote:
> Hello Craig:
> I want to know a sequential scan( full scan table) it can read how
> many blocks every io?
> I know scan index read is one block every io.
>
> Oracle parameter db_file_multiblock_read_count it can control
> scattered read (full scan table),
> assume db_file_multiblock_read_count =10,when scan full table,every
> io request blocks is 10 blocks.
> I know scan index read is one block every io.
>
> So I want to when happen scan full table,postgresql scan full
> table scan how many blocks every io?
PostgreSQL uses the normal operating system facilities for file storage and
access to do its I/O, rather than doing direct unbuffered I/O to files or
even raw volumes like Oracle. It works quite differently, using regular
buffered I/O.
The amount of I/O required for and the I/O patterns of a sequential scan in
PostgreSQL depend on:
- The operating system readahead setting
- Whether the operating system has any of the desired blocks cached
- The file system and I/O scheduler in use
Because of that I'm not really sure your question can be answered, except by
benchmarking and real world measurement. PostgreSQL doesn't really have the
same concept of "an I/O" - though I guess a pread() call would be fairly
close.
What is your goal? What are you trying to achieve? Why are you trying to
determine this? What is the problem you are trying to solve?
--
Craig Ringer
