Hi Sergei, thnkas for your reply. In my tests I found that the first table-scan takes about 12 seconds, the second table-scan takes about 25 seconds (and all folowing) scans too). When I call the function reinit_io_cache() all scans take about 12 seonds. But my implementation was quick and dirty.... I found the reason for this behaviour in the function _mi_read_cache() where the first if()-statement was false in the case of the first scan and true for all other scans.My conclusion was that the IO_CACHE-struct ure had to be reset when the second scan begins. And this seems to work. Regards AugustQ Am Montag, den 30.01.2017, 12:27 +0100 schrieb Sergei Golubchik: > 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 secur...@mariadb.org
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