Date: Thu, 03 Jul 2008 14:56:27 +0200
To: =?utf-8?Q?Lev=C3=AD_Teodoro_da_Silva?= <tlevisilva@gmail.com>,
	"pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Subject: Re: [QUESTION]Concurrent Access
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> I want to know if the PostGree has limitations about the concurrent  =

> access,
> because a lot of people will access this database at the same time.

	PostgreSQL has excellent concurrency provided you use it correctly.

	But what do you mean by concurrent access ?

	* Number of opened Postgres connections at the same time ?
	=3D> each one of those uses a little bit of RAM. (see manual) but if id=
le  =

they don't use CPU.

	* Number of opened transactions at the same time ?
	(between BEGIN and COMMIT)
	If your transactions are long and you have many transactions at the sam=
e  =

time you can get lock problems, for instance transaction A updates row X=
  =

and transaction B updates the same row X, one will have to wait for the =
 =

other to commit or rollback of course. If your transactions last 1 ms  =

there is no problem, if they last 5 minutes you will suffer.

	* Number of queries executing at the same time ?
	This is different from above, each query will eat some CPU and IO  =

resources, and memory too.

	* Number of concurrent HTTP connections to your website ?
	If you have a website, you will probably use some form of connection  =

pooling, or lighttpd/fastcgi, or a proxy, whatever, so the number of ope=
n  =

database connections at the same time won't be that high. Unless you use=
  =

mod_php without connection pooling, in that case it will suck of course,=
  =

but that's normal.

	* Number of people using your client ?
	See number of idle connections above. Or use connection pool.

> I want to know about the limitations, like how much memory do i have t=
o  =

> use

	That depends on what you want to do ;)

> How big could be my database ?

	That depends on what you do with it ;)

	Working set size is more relevant than total database size.

	For instance if your database contains orders from the last 10 years, b=
ut  =

only current orders (say orders from this month) are accessed all the  =

time, with old orders being rarely accessed, you want the last 1-2 month=
s'  =

worth of orders to fit in RAM for fast access (caching) but you don't ne=
ed  =

RAM to fit your entire database.
	So, think about working sets not total sizes.

	And there is no limit on the table size (well, there is, but you'll nev=
er  =

hit it). People have terabytes in postgres and it seems to work ;)