Re: Question about the Internals

Relaxin wrote:
> I'm a consultant for a large company and they are looking at a database to
> use.
>
> I made the suggestion about going with Postgresql.
>
> I told him about it's MVCC design, he liked that ideal, but wanted to know
> exactly HOW did it handle the multiple versions of records.  He's concerned
> because he was burn very early on by another database that was MVCC (won't
> mention any names) and his system became corrupt.  That database has since
> improved and fixed all of their problems.
>
> But the question is, how does Postgresql handle the multiple versions?
> How does it handle the records BEFORE they are committed and how does it
> handle the records AFTER they are committed that allows different users to
> possible have a different view of the data and indexes. And also how does
> the transactional part of the system place a role?

Each record has a transction id of creation and an expire transaction
id.  When you add a record, you put your xid on the old record and
create a new one.  No one has reported any corruption problems with our
database for a long time, so you should be fine.

If you want details, see the developers web page and see my internals
presentation --- it has a diagram showing old/new rows and their
visibility to a single transaction.

--
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman@candle.pha.pa.us               |  (610) 359-1001
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