Hey Zhipan, 2010/11/27 Zhipan Wang <wzhi...@soe.ucsc.edu>
> Hi, > > I want to access part of a table on the disk sequentially, i,e., when I get > to a tuple in the table, I need to read several pages of data in the table > starting from this tuple. I guess CTID could be translated to physical > address on the disk to retrieve this tuple, right? If so, how do I use CTID > to retrieve a particular tuple (or a page) in SQL? Can I use OID to do this > equally efficiently? > Consider to use cursors to read sequentially by FETCH. http://www.postgresql.org/docs/9.0/static/sql-fetch.html > > Another question is: when I update a tuple in a table, this tuple will get > a new CTID and it leaves a gap at the old CTID, and when I insert a new > tuple, it's appended to the end of the table, so the gap is always there. > Does this mean it actually inserts a new tuple and the out-dated tuple still > occupies the space? How can I write the updated tuple back to its original > position to utilize disk space more efficiently? > I believe that VACUUM works well on it. http://www.postgresql.org/docs/9.0/static/sql-vacuum.html > > Thanks! > > -- > Sent via pgsql-general mailing list (pgsql-general@postgresql.org) > To make changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-general > -- // Dmitriy.
