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Re: an observation


hum... did you checked only linux version, or windows too?
could you provide a SQL example?

2017-01-29 14:30 GMT-02:00 AugustQ <augustq@xxxxxxx>:
> 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.
> 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.
> I assume the engine Maria will show the same effect (I didn't check other
> engines).
> Some questions: is my observation correct? Do you think this needs to be
> fixed? Or is there no further development on this part of the code?
> Thanks
> Regards
> AugustQ
> PS: you will find more details on this topic here:
> http://augustq.blogspot.com/2017/01/subselect-execution-3.html
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Roberto Spadim
SPAEmpresarial - Software ERP
Eng. Automação e Controle

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