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@gmx.net>:
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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