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Re: DBD::PostgreSQL

From: Tim Bunce <Tim(dot)Bunce(at)pobox(dot)com>
To: David Wheeler <david(at)wheeler(dot)net>
Cc: Rudy Lippan <rlippan(at)remotelinux(dot)com>, dbi-dev(at)perl(dot)org,pgsql-interfaces(at)postgresql(dot)org
Subject: Re: DBD::PostgreSQL
Date: 2002-11-18 17:44:08
Message-ID: 20021118174408.GF1444@dansat.data-plan.com (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-interfaces
On Mon, Nov 18, 2002 at 08:42:01AM -0800, David Wheeler wrote:
> On Sunday, November 17, 2002, at 08:26  PM, Rudy Lippan wrote:
> 
> >I would guess this is along the lines of std PostgeSQL behaviour; when 
> >you
> >begin_work you tell the db to start a transaction (BEGIN) up until the
> >next commit/rollback.  So instead of turning autocommit off you can 
> >just
> >begin work around the blocks of code that need transactions.  (cf. 
> >local
> >($dbh->{AutoCommit}) = 0)
> 
> Okay, so if I understand correctly, it's an alternative to AutoCommit 
> for handling transactions. That explains why they *both* need to be checked.

Just to be sure this is clear, begin_work is *just* an alternative
way to set AutoCommit off till the next commit or rollback. The
application is free to just set AutoCommit explicitly as needed.

The *only* time a driver needs to consider the BegunWork attribute
is immediately after a commit or rollback.

> >dbd_preparse scans and rewrites the query for placeholders, so if you
> >want to use placeholders with prepare, you will need to walk the string
> >looking for placeholders. How do you think DBD::Pg knows that when you
> >say $sth = $x->prepare("SELECT * FROM thing WHERE 1=? and 2 =?) that 
> >$sth
> >is going to need two placeholders when execute() is called?
> 
> Right, okay, that's *kind of* what I thought. It just seems a shame 
> that each query has to be parsed twice (once by the DBI driver, once by 
> PostgreSQL).

Should need to "parse" in the formal sense, no full grammar is
needed, it's just a very quick skim through the string. If done in
C it should be too cheap to worry about. Especially as it's only
done at prepare() time, not execute().

> >>* One more thing: I was looking at the PostgreSQL documents for the 
> >>new
> >>support for prepared statements in version 7.3. They look like this:
> >>
> >>PREPARE q3(text, int, float, boolean, oid, smallint) AS
> >>	SELECT * FROM tenk1 WHERE string4 = $1 AND (four = $2 OR
> >>	ten = $3::bigint OR true = $4 OR oid = $5 OR odd = $6::int);
> >>
> >From my rough scanning of the docs a few weeks ago, I think that the
> >types are optional (I hope that thy are, in any event), & you are
> >missing the plan_name.
> 
> Unfortunately, according to Tom Lane, the data types are required. :-(
> FWIW with the above example, I swiped it right out of PostgreSQL's 
> tests. the plan_name is "q3".
> 
> >You do not want to go there (trying to magically get the types for the
> >placeholders (unless PostgreSQL will give them to you)).
> 
> Not easily, I think. A shame, really, that the data types are required, 
> as it means that dynamic database clients like DBI (and, I expect, 
> JDBC) won't really be able to take advantage of prepared statements. 
> Only custom code that uses the PostgreSQL API directly (that is, C 
> applications) will be able to do it.

Would binding a string type to an integer (etc) placeholder work?
If so, just ignore the types and bind everything as strings.
That's exactly what DBD::Oracle does.

Tim.

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