Re: general questions on Postgresql and deployment on win32 platform - Mailing list pgsql-general

> I have gone through the documentation that come with version
> 8 beta 4 and I have a number of questions.
>
> (1) backup/restore
> I notice that in the documentation, it seems to suggest that
> an online backup, made via pg_start_backup() and
> pg_stop_backup() functions would back up all databases
> running on the server, rather than any specific instance.
> If that's the case, is it correct to state that WAL is
> instance specific rather than database specific?
>
> Does it also mean that I must back up and restore all the
> databases (or database cluster in Postgresql Speak) even if I
> am only interested in 1 database?

If you want to use PITR based backup/recovery, yes. If you use the
"classic" way of doing pg_dump of the database, then you can do it on
individual database location.


> (2) WAL location
> I can't seem to find a way to move WAL log to a different
> location other than data/pg_xlog. On *nix platform, it's a
> simple matter of creating a symbolic link. But on win32,
> there is no equivalent. However, even under *nix system, I
> believe symbolic link can only be created for directories on
> the same hard drive. This seems less than optimal. Typically,
> one would place database files on RAID 5 drives (to maximize
> random access speed) and log files on mirrored drives (to
> maximize sequential access speed).

You can use either Directory Junctions (use the junction tool from
www.sysinternals.com for example), or you can mount the new disk
directly into the WAL directory (this can be done using the disk
administrator mmc snapin, or using the diskpart.exe utility)


> (3) Trigger
> Is it correct to state that old and new keywords are only
> available to PL/pgSQL function (but not a SQL function) and
> to row level trigger? Is there anyway to refer to old and new
> rows for statement level trigger? The visibility rule is also
> quite confusing. Can someone please clarify it in terms of
> before/after and row/statement level triggers (i.e., 4
> possible permutations).

Can't speak for this one specifically.


> (4) Process/thread on win32 platform
> On win32 platform, a process simply provides environment for
> a thread to run (whereas a thread is more like a *nix
> process, win32 has something called fibre that is more
> lightweight than thread). Typically, it's not recommended to
> start a new process per connection for DBMS running on win32
> lest its performance would suffer. For example, SQL Server
> actually spawns threads to manage connections rather than
> processes. Does the Postgresql on win32 platform use process
> or thread when a connection is made?

Postgresql on win32 uses a process/connection model. This is far from
ideal, but it's done to make sure you actually use the same database as
you do on Unix.
You can regain a large part of the loss by using pooled connections.

(BTW, SQL Server doesn't necessarily use a thread/connection model
either - it can use both fibres and some other hybrids. But you're right
in that it only uses one process)


> I notice that the
> configuration file lets you specify maximum number of
> connections. If I have a very high number of persistent
> concurrent users (thus preventing connection pooling from
> helping much), would performance suffer significantly?

It would suffer some, but not very much I think. It uses more shared
memory, but that's not a big issue on win32. Some scans have to be made
over larger memory areas, and that's where you get a loss. I don't
*think* it would be significant, but no testing has been done for that
AFAIK.

//Magnus