Gotcha. Well ... instead of a $dbh->do(...) ... which would read all the rows into memory before returning, I'd try the prepare/execute/fetchrow approach and hope the underlying DBD/OCI does the right thing memory wise.

  my $sth = $dbh->prepare( $ctas_sql );
     $sth->execute();

  while( my $row = $sth->fetchrow_arrayref ) {
     # do whatever with $row
  }
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Update: Doh! just reread this and saw it was a create statement, not a select. In that case I'm guessing you're timing out somewhere. You may want to look into using DBI->trace to try and track that down.