Batch Fetching

Batch fetching is the ability to fetch the entire result set from an SQL query in one call, as opposed to iterating through the result set using row-oriented methods such as fetchrow_array( ), etc.

The DBI defines several methods for this purpose, including fetchall_arrayref( ) and selectall_arrayref( ) , which basically retrieve the entire result set into a Perl data structure for you to manipulate.

fetchall_arrayref( ) operates in three different modes depending on what arguments have been passed to it. It can be called with no arguments, with a reference to an array slice as an argument, and with a reference to a hash slice as an argument. We’ll discuss these modes in the following sections.

No arguments

When fetchall_arrayref( ) is invoked with no arguments, it returns a reference to an array containing references to each row in the result set. Each of those references refers to an array containing the field values for that row. Figure 5.4 illustrates the data structure returned.

Figure 5-4. fetchall_arrayref( ) data structure

This looks pretty convoluted, but it is, in fact, extremely simple to access the data stored within the data structure. For example, the following code shows how to dereference the data structure returned by fetchall_arrayref( ) when run with no arguments:

#!/usr/bin/perl -w
#
# ch05/fetchall_arrayref/ex1: Complete example that connects to a database,
#                             executes a SQL statement, then fetches all the
#                             data rows out into a data structure. This
#                             structure is then traversed and printed.

use DBI;

### The database handle
my $dbh = DBI->connect( "dbi:Oracle:archaeo", "username", "password" , {
    RaiseError => 1
});

### The statement handle
my $sth = $dbh->prepare( " SELECT name, location, mapref FROM megaliths " );

### Execute the statement
$sth->execute(  );

### Fetch all the data into a Perl data structure
my $array_ref = $sth->fetchall_arrayref(  );

### Traverse the data structure and dump each piece of data out
###
### For each row in the returned array reference ...
foreach my $row (@$array_ref) {
    ### Split the row up and print each field ...
    my ( $name, $type, $location ) = @$row;
    print "\tMegalithic site $name, found in $location, is a $type\n";
}

exit;

Therefore, if you want to fetch all of the result set from your database, fetchall_arrayref( ) is an efficient and easy way of doing it. This is doubly true if you were planning on building an in-memory data structure containing the returned rows for post-processing. Instead of doing it yourself, you can simply use what fetchall_arrayref( ) returned instead.