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rows affected inside a transaction

by x5150 (Acolyte)
on May 13, 2008 at 20:56 UTC ( #686366=perlquestion: print w/ replies, xml ) Need Help??
x5150 has asked for the wisdom of the Perl Monks concerning the following question:

My problem is sth->rows always returns zero for a DELETE command inside a transaction. I'm using DBD::Pg 2.7.1.
$dbh->begin_work; foreach (@$bp_aref) { $bind_parms = $_; my $ra = $sth->execute(@$bind_parms); # $ra is zero # records are still in the db # I need to keep a count of how many rows are going to be deleted, for + each delete statement $count[$i] += $sth->rows; # returns zero } $dbh->commit unless $skip_trans; # now records are not in db
How can I get a number of rows affected inside the transaction? Thanks.

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Re: rows affected inside a transaction
by gam3 (Curate) on May 14, 2008 at 03:40 UTC
    I ran this code and ->rows was never zero. I ran it on the data that is at the bottom.
    use DBI; my $db = 'Pg'; my $database = 'test'; my $user = 'test'; my $password = 'test'; my $dbh = DBI->connect( "DBI:${db}:database=" . $database . ";", $user, $password, { RaiseError => 1, PrintError => 1, PrintWarn => 1, # AutoCommit => 0, } ); $dbh->begin_work; my $st = $dbh->prepare('delete from test where inc = ?'); for my $x (2, 4, 6) { my $ra = $st->execute($x); print $st->rows, "\n"; } $dbh->rollback;
    The output of this is:

    Here is the a dump of the database that I used.

    -- -- PostgreSQL database dump -- SET client_encoding = 'UTF8'; SET check_function_bodies = false; SET client_min_messages = warning; -- -- Name: SCHEMA public; Type: COMMENT; Schema: -; Owner: postgres -- COMMENT ON SCHEMA public IS 'Standard public schema'; CREATE USER test WITH PASSWORD 'test'; SET search_path = public, pg_catalog; SET default_tablespace = ''; SET default_with_oids = false; -- -- Name: test; Type: TABLE; Schema: public; Owner: test; Tablespace: -- CREATE TABLE test ( inc integer, test character varying(40) ); ALTER TABLE public.test OWNER TO test; -- -- Data for Name: test; Type: TABLE DATA; Schema: public; Owner: test -- COPY test (inc, test) FROM stdin; 1 a 2 b 3 b 4 b 5 b 6 c \. GRANT ALL ON TABLE test TO test;

    You might want to run a simple test like this to check that the problem is in DBI and not in your logic.

    -- gam3
    A picture is worth a thousand words, but takes 200K.
Re: rows affected inside a transaction
by kyle (Abbot) on May 14, 2008 at 03:42 UTC

    This is not an ideal solution, but...

    Since you're in a transaction, you don't have to worry about a race condition. As such, you can query for the matching records before you delete them.

    my $suffix = 'FROM table WHERE blah blah'; my ($to_be_deleted) = $dbh->selectrow_array( "SELECT count(*) $suffix" ); $dbh->do( "DELETE $suffix" );

    The first problem with this is performance. You'll be going to these same records twice. If it's a huge table, and the condition causes a full table scan, this is going to be really painful.

    The second problem is that it's possible for $suffix to work fine for the "SELECT", but not for the "DELETE". That's not a huge problem, I think, because it will abort the transaction. That is, it'll fail instead of lie. I could be wrong, however. Not knowing what kind of SQL you have in $sth, I don't know if this is a potential issue.

      Found out there were some DELETE and SELECT triggers on those tables that didn't actually delete the data but looked like it did.

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