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Programming the Perl DBI

  • April 10, 2003
  • By Developer.com Staff
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Utility Methods and Functions

To round off our basic introduction to DBI, we'll tell you about some useful utility methods and functions that will make your life that little bit easier. These include the very useful quote escaping method, DBI execution tracing, and various functions to tidy up your data.

Database-Specific Quote Handling

By far the most important utility method is quote(), which correctly quotes and escapes SQL statements in a way that is suitable for a given database engine. This feature is important if you have a Perl string that you wish to insert into a database, as the data will be required, in most cases, to have quotation marks around it.

To confuse matters, database engines tend to have a different format for specifying these surrounding quotation marks. DBI circumvents this problem by declaring the quote() method to be executed against a database handle, which ensures that the correct quotation rules are applied.

This method, when executed against a database handle, converts the string given as an argument according to defined rules, and returns the correctly escaped string for use against the database.

For example:

#!/usr/bin/perl -w
#
# ch04/util/quote1: Demonstrates the use of the
# $dbh->quote() method

use DBI;

### The string to quote
my $string = "Don't view in monochrome (it looks 'fuzzy')!";

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

### Escape the string quotes ...
my $quotedString = $dbh->quote( $string );

### Use quoted string as a string literal in a SQL statement
my $sth = $dbh->prepare( "
    SELECT *
    FROM media
    WHERE description = $quotedString
  " );
$sth->execute();

exit;

For example, if you quoted the Perl string of Do it! via an Oracle database handle, you would be returned the value of 'Do it!'. However, the quote() method also takes care of cases such as Don't do it! which needs to be translated to 'Don''t do it!' for most databases. The simplistic addition of surrounding quotes would have produced 'Don't do it!' which is not a valid SQL string literal.

Some databases require a more complex quote() method, and some drivers (though not all) have a quote() method that can cope with multiline strings and even binary data.

As a special case, if the argument is undef, the quote() method returns the string NULL, without quotes. This corresponds to the DBI's use of undef to represent NULL values, and to how NULL values are used in SQL.

Tracing DBI Execution

DBI sports an extremely useful ability to generate runtime tracing information of what it's doing, which can be a huge time-saver when trying to track down strange problems in your DBI programs.

At the highest level, you can call the DBI->trace() method, which enables tracing on all DBI operations from that point onwards. There are several valid tracing levels:

0 Disables tracing.
1 Traces DBI method execution showing returned values and errors.
2 As for 1, but also includes method entry with parameters.
3 As for 2, but also includes more internal driver trace information.
4 Levels 4, and above can include more detail than is helpful.

The trace() method can be used with two argument forms, either specifying only the trace level or specifying both the trace level and a file to which the trace information is appended. The following example shows the use of DBI->trace():

#!/usr/bin/perl -w
#
# ch04/util/trace1: Demonstrates the use of DBI tracing.

use DBI;

### Remove any old trace files
unlink 'dbitrace.log' if -e 'dbitrace.log';

### Connect to a database
my $dbh = DBI->connect( "dbi:Oracle:archaeo", "username",
                        "password" );

### Set the tracing level to 1 and prepare()
DBI->trace( 1 );
doPrepare();

### Set trace output to a file at level 2 and prepare()
DBI->trace( 2, 'dbitrace.log' );
doPrepare();

### Set the trace output back to STDERR at level 2 and prepare()
DBI->trace( 2, undef );
doPrepare();

exit;

### prepare a statement (invalid to demonstrate tracing)
sub doPrepare {
    print "Preparing and executing statement\n";
    my $sth = $dbh->prepare( "
        SELECT * FROM megalith
    " );
    $sth->execute();
    return;
}

exit;

This program generates quite a bit of trace information, of which we'll show just a small fragment:

  -> prepare for DBD::Oracle::db (DBI::db=HASH(0xcd45c)~0xcd4a4 '
      SELECT * FROM megalith
  ') thr0
  <- prepare= DBI::st=HASH(0xcd648) at trace1 line 30.
  -> execute for DBD::Oracle::st 
             (DBI::st=HASH(0xcd648)~0x16afec) thr0
  dbd_st_execute SELECT (out0, lob0)...
  !! ERROR: 942 'ORA-00942: table or view does not exist
     (DBD ERROR: OCIStmtExecute)'
  <- execute= undef at trace1 line 33.
DBD::Oracle::st execute failed: ORA-00942: table or view does
     not exist (DBD ERROR: OCIStmtExecute) at trace1 line 33.

This trace information was generated with a setting of level 2, and shows the operations that DBI undertook when trying to prepare and execute a statement. Lines prepended with -> are written when the method is being entered, and lines prepended with <- are written when the method is returning. These lines also show the information being returned from the method call. The DBI trace output is indented by four spaces to make it easier to distinguish the trace output from any other program output.

You can see the prepare() method being called along with its parameters: a database handle and the SQL statement to prepare.[9] The next line shows the prepare() returning a statement handle. It also shows the file and line number that prepare() was called from. Following that, we see execute() being called, a trace line from the driver itself, and the method returning after logging an error. Finally we see the warning generated by the DBI due to the PrintError attribute, which is on by default.

The trace information generated at level 1 is similar. The main difference is that the method entry lines (->) are not shown.

The one drawback to this form of tracing is that if your program uses a lot of handles, then the volume of tracing information could be quite vast. Similarly, you might have tracked your problem down to a specific database operation that you'd like to trace individually.

The trace() method is also available at a handle level, allowing you to individually trace any database and statement handle operations. Therefore, you could trace operations on a given database handle to level 1 and a single statement handle to level 2. For example:

### Connect to a database...
my $dbh = DBI->connect( "dbi:Oracle:archaeo", "username",
                        "password" );

### Trace the database handle to level 1 to the screen
$dbh->trace( 1 );

### Create a new statement
my $sth = ...;

### Trace the statement to level 2 to the file 'trace.lis'
$sth->trace( 2, 'trace.lis' );

Note that if a filename is specified when calling trace(), then currently, trace output from all handles is redirected to that file.

If your programs are exhibiting odd behavior or are generating errors on a regular basis, you should consider using the built-in tracing features of DBI to help you resolve your problems. This tool is extremely useful, as you will be able to see exactly what data is being passed to the database, allowing you to ensure that it's in the correct format.

Finally, tracing can also be controlled via the use of an environment variable called DBI_TRACE, which acts in a similar manner to the DBI->trace() method. That is, it traces all handles used within the program. This environment variable can be used in three ways that are summarized in the following table.

DBI_TRACE Value

Effect on DBI

1

DBI->trace(1);

dbitrace.log

DBI->trace(2, 'dbitrace.log');

4=dbitrace.log

DBI->trace(4, 'dbitrace.log');

If the trace level isn't specified in the DBI_TRACE environment variable, it will default to 2, as shown in the table above.





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