DB::Batch - Run database queries in batches through DBI

DB::Batch is a system for breaking up queries into batches behind the scenes, 
while maintaining the appearance of running a single query.  
Queries are run against an instance of DBI.  

For example, a query like: 

SELECT * FROM TABLE

is expressed as:

SELECT * FROM TABLE WHERE id BETWEEN # AND #

and would be executed as:

SELECT * FROM TABLE WHERE id BETWEEN 1 AND 100
SELECT * FROM TABLE WHERE id BETWEEN 101 AND 200
SELECT * FROM TABLE WHERE id BETWEEN 201 AND 300
...

Until the MAX(id) is reached.

The script running this query, however, acts as if you are just 
running 'SELECT * FROM TABLE'.

The purpose of this is to diminish load on a sql database when large amounts of
data need to be retrieved by managing batch size and throttling.  

Divide a query over a primary key.

In this example, we use a BETWEEN clause with two '#' placeholders. During execution the values in these placeholders are automatically incremented in chunks of 1000 (based on the batch size sent in) The execution starts at the given start and end values, which can be defined as integers or sql statements

calling $batch->fetchrow_array, fetchrow_hashref, or fetchrow_array will return all rows from the table, however in the background it will be running separate queries for each batch

use DB::Batch;
use DBI;

my $dbh   = DBI->connect(...);
my $batch = DB::Batch->new(dbh => $dbh);

my $sql = 'SELECT id,col FROM table WHERE id BETWEEN # AND #';
my %db_args = (
        start => 1,
        end   => 'SELECT MAX(id) FROM table',
        batch => 1000,
        sleep => 1,
);

# this will retrieve all rows from table
while (my ($id,$col) = $batch->fetchrow_array($sql,\%db_args) {
        print "$id,$col \n";
}
        

Divide a query using limits and offsets

In this example, DB::Batch will automatically increment our values for LIMIT and OFFSET behind the scenes until 1000 rows have been returned. (Note: this method is not optimal for very large data sets)

use DB::Batch;
use DBI;

my $dbh   = DBI->connect(...);
my $batch = DB::Batch->new(dbh => $dbh);

my $sql = 'SELECT id,col FROM table WHERE id LIMIT # OFFSET #';

# you can also use the form:      
# my $sql = 'SELECT id,col FROM table WHERE id LIMIT #,#';

my %db_args = (
        start => 1,
        limit => 1000, 
        batch => 100,
        sleep => 1,
);

# this will retrieve all rows from table
while (my ($id,$col) = $batch->fetchrow_array($sql,\%db_args) {
        print "$id,$col \n";
}
        

Selecting with a list of arguments.

In this example we provide a list of arguments and tell DB::Batch to bind 10 values at a time. The '#' will be expanded to the number of placeholders specified by makebinds, or up to as many placeholders that are needed to bind all the values of the given list.

use DB::Batch;
use DBI;

my $dbh   = DBI->connect(...);
my $batch = DB::Batch->new(dbh => $dbh);

my $sql = 'SELECT id,col FROM table WHERE id IN (#)';


my %db_args = (
        list      => [qw(1..105)],
        makebinds => 10,
);

# this will retrieve all rows from table
while (my ($id,$col) = $batch->fetchrow_array($sql,\%db_args) {
        print "$id,$col \n";
}

Inserting.

Inserting can be done in batches using do_batch. In this example, 10 values from the given list will be binded and executed at a time behind the scenes.

use DB::Batch;
use DBI;

my $dbh   = DBI->connect(...);
my $batch = DB::Batch->new(dbh => $dbh);

my $sql   = "UPDATE foo SET col='abc' WHERE id IN (#)";

my %db_args = (
        list      => [qw(1..105)],
        makebinds => 10,
);

# this will retrieve all rows from table
$batch->do_batch($sql,\%db_args);

Inserting II

inserting multiple rows at once can be done by specifying the number of groups and number of placeholders with makebinds

use DB::Batch;
use DBI;

my $dbh   = DBI->connect(...);
my $batch = DB::Batch->new(dbh => $dbh);

my $sql = 'INSERT INTO foo (col1,col2) VALUES #';

my %db_args = (
        list      => [qw(1 col2value2 2 col2value2 3 col2value3)],
        makebinds => 2, #
        groups    => 3, # make 3 groups of 2 placeholders
);

# this will retrieve all rows from table
$batch->do_batch($sql,\%db_args);

Inserting III

If the flow of control of your script requires you to iterate over a large amount of data, use buffer_batch to buffer a number of rows of data and then execute it all at once in the end

use DB::Batch;
use DBI;

my $dbh   = DBI->connect(...);
my $batch = DB::Batch->new(dbh => $dbh);

my $sql = 'INSERT INTO foo (col1,col2) VALUES #';

my @data = (1..10000);

my %db_args = (
        makebinds => 2, #
        groups    => 3, # make 3 groups of 2 placeholders
);

for my $col1 (@data) {
        my $col2 = $col1;
        $batch->buffer_batch($sql,$col1,$col2);
}         

$batch->exec_buffer($sql,\%db_args);

Hooks

use DB::Batch;
use DBI;

my $dbh   = DBI->connect(...);
my $batch = DB::Batch->new(dbh => $dbh);

my $sql    = 'SELECT id,col FROM table WHERE id BETWEEN # AND #';
my $insert = 'INSERT INTO foo (col1,col2) VALUES #';


my %db_args = (
        start => 1,
        end   => 'SELECT MAX(id) FROM table',
        batch => 1000,
        sleep => 1,
        pre_hook => sub {
                # this is executed before the query is run through $dbh->prepare
                print Dumper [ $batch->get_last_batch() ]; # returns values about to be bound
        },
        post_hook => sub {
                # This will run the INSERT after each batch of the SELECT query is finished,
                # this way the entire table worth of data doesn't need to be kept in memory 
                # and written all at once
                $batch->exec_buffer($insert,{makebinds => 2, groups => 10});
        }
);


# this will retrieve all rows from table
while (my ($id,$col) = $batch->fetchrow_array($sql,\%db_args) {

        # returns true if a new batch was started behind the scenes
        if ($batch->is_new_batch()) {
                print Dumper [ $batch->get_last_batch() ]; # returns current values being bound
        }

        # you can buffer results from one query to insert in batches to another table
        $batch->buffer_batch($insert,$id,$col);
}

new (dbh => DBI::db)

fetchrow_array ($SQL_STRING, {ARGS})

fetchrow_arrayref ($SQL_STRING, {ARGS})

fetchrow_hashref ($SQL_STRING, {ARGS})

-- start at id 0, and run the query in increments of 100 up to 1000.  
SQL_STRING: 'SELECT foo FROM table WHERE id BETWEEN # AND #'
ARGS: {
        start => 0
        end   => 1000,
        batch => 100
        sleep => 100000  # number of microseconds to wait between queries
        pre_hook => sub {
                # do something prior to current sth being prepared
        },
        post_hook => sub {
                # do something after current sth is finished
        }
}
     
-- determine start and end values at runtime based on the results of the given queries
ARGS: {
        start => 'SELECT MIN(id) FROM table'
        end   => 'SELECT MAX(id) FROM table',
        batch => 100
}
     
     
-- expand the placeholder into 10 placeholders and bind the values of 'list' 10 at a time
SQL_STRING: 'SELECT foo FROM table WHERE id IN (#)'
ARGS: {
        makebinds => 10,
        list      => [qw(2 4 6 8 10 12 14 16 18 20)],
}

buffer_batch ($SQL_STRING,@LIST)

buffer the values in @LIST to be bound and run when exec_buffer is called on the same SQL_STRING

exec_buffer($SQL_STRING,\%ARGS)

run do_batch after having buffered a list of bind values using buffer_batch();

     -- args are identical to do_batch, except 'list' was build internally when buffer_batch was called
SQL_STRING: 'INSERT INTO table VALUES #'
ARGS: {
        makebinds => 10,
}

buffer_batch_triggered ($SQL_STRING,\%ARGS,@LIST)

buffer the values in @LIST to be bound and run when exec_buffer is called on the same SQL_STRING
when the number of rows of data matches $ARGS->{trigger}, the insert is executed automatically

do_batch

execute write queries in batches

SQL_STRING: 'INSERT INTO table VALUES #'
ARGS: {
        makebinds => 3,
        groups    => 2
}

get_dbh

return the primary db handle being used

get_last_batch

return a list of the last batch values bound to the current query

get_last_query

return a string with the full query that was last executed.

get_sth

call this from inside a while($batch->fetch...) { } loop.  Return the statement handle currently being processed

is_new_batch

call this from inside a while($batch->fetch...) { } loop.  Returns 1 if a new query batch was executed.

Copyright 2010, Chris Becker <[email protected]>

Original work sponsered by Shutterstock, LLC. http://shutterstock.com

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.