> -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of > Bruce Momjian > Sent: Friday, August 27, 2004 1:27 PM > To: Adi Alurkar > Cc: [EMAIL PROTECTED] > Subject: Re: [PERFORM] Equivalent praxis to CLUSTERED INDEX? > > > > But what is the advantage of non-full pages in Oracle? >
One advantage has to do with updates of variable-length columns, e.g. varchars. If the block is fully packed with data, an update to a varchar column that makes the column wider, causes "row-chaining". This means that a portion of the row is stored in a different data block, which may be somewhere completely different in the storage array. Retrieving that row (or even just that column from that row) as a unit may now require additional disk seek(s). Leaving some space for updates in each data block doesn't prevent this problem completely, but mitigates it to a certain extent. If for instance a row is typically inserted with a null value for a varchar column, but the application developer knows it will almost always get updated with some value later on, then leaving a certain percentage of empty space in each block allocated to that table makes sense. Conversely, if you know that your data is never going to get updated (e.g. a data warehousing application), you might specify to pack the blocks as full as possible. This makes for the most efficient data retrieval performance. - Jeremy ---------------------------(end of broadcast)--------------------------- TIP 7: don't forget to increase your free space map settings
