Re: Per-session data?

Steve Atkins wrote:

> I have an application where each user session opens and maintains a
> long-lived connection to the postgresql backend database.
>
> I need to keep a small amount of information (things like a unique
> session identifier, the application - as opposed to database - username
> and so on) that is local to each database session. It needs to be
> visible from within plpgsql trigger functions and will be used
> on a large fraction of updates.
>
> I can see a few ways of doing it, none of them terribly pretty:
>
>   Keep all the data in a globally visible table, indexed by the
>   PID of the database backend.
>
>   Create a temporary table at the beginning of each session containing
>   the data, and simply read it out of that, relying on the temporary
>   table to be session-local.
>
> Anyone have a suggestion for something that's either prettier, lower
> overhead or both?

One way to do this is to write a C-language function to set a global
variable and another to read from that variable. I.e. write a:

void setCookie(text);
text getCookie();

pair, invoking setCookie() upon connecting to the database.

There are three problems with using PostgreSQL temporary tables:

1. PL/pgSQL will cache the OID of the temporary table that existed
when it is first parsed, and when that temporary table is dropped
and recreated later, despite having the same name and structure,
you'll get an error like:

ERROR:  relation with OID 869140 does not exist
CONTEXT:  PL/pgSQL function "mytest" line 4 at select into variables

The work-around is to use EXECUTE to build the query string at
run-time.

2. For the same reason as above, you cannot build views against the
session-local temporary table. However, you could write a wrapper
PL/pgSQL function that leverages EXECUTE and use that wrapper
function in your view definition.

3. Under pre-7.4 databases, the large number of temporary table
creations/drops caused system catalog index bloat, which required
occassional REINDEXing under a stand-alone postgres backend. Under
7.4's index reuse, that seems to have abated.

You can build both PL/pgSQL functions and VIEWs against your 'C'
functions though. I suspect performance would be better, but you'd
have to do some testing. In the long run, I assume PostgreSQL will
one day have SQL temporary tables (whose structure persists across
sessions but whose content does not) which will ultimately be the
correct solution. Until then, it's a matter of preference and
hoop-jumping depending upon server version...

HTH,

Mike Mascari