如何使用perl和postgresql遍历大型结果集

Question

用于Perl的DBD::Pg PostgreSQL绑定将始终获取查询的整个结果集。因此，如果您使用简单的准备执行来遍历大型表，那么只需运行$sth->execute()就可以在内存中找到整个表。像fetch_row这样准备好的陈述和电话也无济于事。

如果您正在使用BIG表，以下将失败。

use DBI;
my $dbh =   DBI->connect("dbi:Pg:dbname=big_db","user","password",{
        AutoCommit => 0,
        ReadOnly => 1,
        PrintError => 1,
        RaiseError =>  1,
});

my $sth = $dbh->prepare('SELECT * FROM big_table');
$sth->execute(); #prepare to run out of memory here
while (my $row = $sth->fetchrow_hashref('NAME_lc')){
  # do something with the $row hash
}
$dbh->disconnect();

Answer 1

要解决此问题，请声明游标。然后使用游标获取数据块。 ReadOnly和AutoCommit设置对于此工作非常重要。由于PostgreSQL只会进行CURSORS阅读。

use DBI;
my $dbh =   DBI->connect("dbi:Pg:dbname=big_db","user","password",{
        AutoCommit => 0,
        ReadOnly => 1,
        PrintError => 1,
        RaiseError =>  1,
});

$dbh->do(<<'SQL');
DECLARE mycursor CURSOR FOR
SELECT * FROM big_table
SQL

my $sth = $dbh->prepare("FETCH 1000 FROM mycursor");
while (1) {
  warn "* fetching 1000 rows\n";
  $sth->execute();
  last if $sth->rows == 0;
  while (my $row = $sth->fetchrow_hashref('NAME_lc')){
    # do something with the $row hash
  }
}
$dbh->disconnect();