* Stefan Hinz > * Sergei: > > It's because of NULLs. > > Extra byte is used to mark, if the column is NULL or not. > > Thanks alot for the explanation. Thanks to Gerald Clark, too, who told > me the same. > > > (in fact it's a bitmap, that is it would be one byte for up to 8 > > columns, then it'll be 2 bytes, etc.) > > If so, this table should have an average row length of 92, not 91, > shouldn't it (9 cols * 10 bytes + 2 bytes)? > > MySQL>DESCRIBE myrowisam; > +-------+----------+------+-----+---------+-------+ > | Field | Type | Null | Key | Default | Extra | > +-------+----------+------+-----+---------+-------+ > | col1 | char(10) | | | | | > | col2 | char(10) | | | | | > | col3 | char(10) | | | | | > | col4 | char(10) | | | | | > | col5 | char(10) | | | | | > | col6 | char(10) | | | | | > | col7 | char(10) | | | | | > | col8 | char(10) | | | | | > | col9 | char(10) | | | | | > +-------+----------+------+-----+---------+-------+
These columns are all defined as NOT NULL, the mentioned bitmap does not apply. The extra byte you see is a deletion flag, used internaly when you delete a record from a fixed width table. The unused 7 bits of this byte is used for the bitmap when you use NULLs, allowing up to 15 NULL fields to be stored with only one additional byte per record: 15 char(10) fields would give a Avg_row_length of 152, while 16 fields would give a Avg_row_length of 163. If there are fewer than 8 NULL fields, no additional byte is needed for the bitmap, leaving the Avg_row_length at the combined field length + 1 for the deletion flag. -- Roger --------------------------------------------------------------------- Before posting, please check: http://www.mysql.com/manual.php (the manual) http://lists.mysql.com/ (the list archive) To request this thread, e-mail <[EMAIL PROTECTED]> To unsubscribe, e-mail <[EMAIL PROTECTED]> Trouble unsubscribing? Try: http://lists.mysql.com/php/unsubscribe.php
