Re: DBD::PostgreSQL

On Sunday, November 17, 2002, at 08:26 PM, Rudy Lippan wrote:

> Current behaviour sounds about right. Iff you are not in auto commit
> mode,
> you have to tell pg to start a new transaction. IIRC, some DBs will
> automatically start a new transaction when the commit/rollback is
> called;
> however, for pg, an explicit BEGIN is required to start the
> transaction.

With feedback from Tom Lane, I think I'll add code to track when to
BEGIN a transaction, and check it in execute() to see if it needs to be
turned on before executing a statement.

> 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.

> dbd_db_commit() returns zero when NULL == $imp_dbh->conn or on error.
> It
> returns one when when PGRES_COMMAND_OK == status.

Okay.

> Humm intersting... It look like the data can be committed to database &
> dbd_db_commit can still through an error because the BEGIN failed.
> Ugg.
> This could be non-pretty.

Yeah, that's another reason to set a flag and remove the BEGIN from
dbd_db_commit() and dbd_db_rollback().

> A transaction is already in progress because you have called BEGIN.

Yes, but if I set the flag as I've mentioned above, I may not have. It
makes sense to me to use the init_commit flag for this purpose.

> Don't know, but it looks like (cursory glance) that dbd_db_disconnect
> gets
> called already before dbd_db_destory in DESTROY of Driver.xst. But hey
> can't hurt, right :)

Um, yes, I guess that's true. I was thinking about redundant operations
using more time, but I guess that doesn't really matter in
dbd_db_destroy() (and it takes next to no time, anyway).

> 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). But I guess there's no other way about it. Perhaps our
preparsed statement could be cached by prepare_cached(), so that, even
though we can't cache a statement prepared by PostgreSQL (see my
exchange with Tom Lane), we could at least cache our own parsed
statement.

>> * 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.

Regards,

David

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