maria-developers team mailing list archive

[Sergei Golubchik <serg@xxxxxxxxxxx>]

Hi, AugustQ!

On Jan 29, AugustQ wrote:
> Hi,
> 
> by playing with the code I think I found something interesting.
> 
> My environment: MariaDB 10.0.10, MyISAM-engine
> 
> I played with a table-scan, no index is defined on this table. When I
> execute a SQL-statement that forces the server to do a second table-
> scan on a table this 2nd table-scan will be slow. 
> 
> The reason for this behaviour is the usage of a buffer: during the 1st
> scan this buffer is filled, used and filled again until the whole
> table is processed. At the end of the 1st scan it contains the last
> bytes of the file. When a 2nd scan is started the reading of the table
> starts from the beginning of the file but the buffer and all
> associated variables are not reset:  the buffer still contains the
> bytes from the end of the file, the request cannot be fulfilled by the
> buffer so the request has to be handled by reading the bytes directly
> from the file  using the read()-function of the Std-library. This
> takes much more time then simply copying the bytes from the internal
> buffer.

Right... MyISAM does not know how you're going to access the table. It
might be a second full table scan. Or may be you'll just want to read
the end of the table?

> My idea is: somewhere in the code this situation must be detected and
> the buffer (and all associated variables) reset to initial
> values. reinit_io_cache() looks like the right candidate for this.

How would that help? You'll get faster execution if MyISAM would preload
first pages of the table. But it doesn't know you're going to do a full
table scan, so why would it preload it?

Regards,
Sergei
Chief Architect MariaDB
and security@xxxxxxxxxxx