DBI Logging and Profiling

If you use Perl and connect to traditional relational databases, you use DBI. Most of the Perl shops I know of nowadays use DBIx::Class to interact with a database. This blog post is how I “downported” some of my DBIx::Class ideas to DBI. Before I say much more I have to thank my boss Bill Hamlin, for showing me how to do this.

Ok so when debugging queries, with DBIx::Class you can set the DBIC_TRACE environment variable and see the queries that the storage layer is running. Sadly sometimes the queries end up mangled, but that is the price you pay for pretty printing.

You can actually get almost the same thing with DBI directly by setting DBI_TRACE to SQL. That is technically not supported everywhere, but it has worked everywhere I’ve tried it. If I recall correctly though, unlike with DBIC_TRACE, using DBI_TRACE=SQL will not include any bind arguments.

Those two features are great for ad hoc debugging, but at some point in the lifetime of an application you want to count the queries executed during some workflow. The obvious example is during the lifetime of a request. One could use DBIx::Class::QueryLog or something like it, but that will miss queries that were executed directly through DBI, and it’s also a relatively expensive way to just count queries.

The way to count queries efficiently involves using DBI::Profile, which is very old school, like a lot of DBI. Here’s how I got it to work just recording counts:

#!/usr/bin/env perl

use 5.12.0;
use warnings;

use Devel::Dwarn;
use DBI;
use DBI::Profile;
$DBI::Profile::ON_DESTROY_DUMP = undef;

my $dbi_profile = DBI::Profile->new(
  Path => [sub { $_[1] eq 'execute' ? ('query') : (\undef) }]

$DBI::shared_profile = $dbi_profile;

my $dbh = DBI->connect('dbi:SQLite::memory:');
my $sth = $dbh->prepare('SELECT 1');

$sth = $dbh->prepare('SELECT 2');

my @data = $dbi_profile->as_node_path_list;
Dwarn \@data;

And in the above case the output is:


The outermost arrayref is supposed to contain all of the profiled queries, so each arrayref inside of that is a query, with it’s profile data as the first value (another arrayref) inside, and all of the values after that first arrayref are user configurable.

So the above means that we ran six queries. There are some numbers about durations but they are so small that I won’t consider them carefully here. See the link above for more information. Normally if you had used DBI::Profile you would see two distinct queries, with a set of profiling data for each, but here we see them all merged into a single bucket. All of the magic for that is in my Path code references.

Let’s dissect it carefully:

$_[1] eq 'execute' # 1
  ? ('query')      # 2
  : (\undef)       # 3

Line 1 checks the DBI method being used. This is how we avoid hugely inflated numbers. We are trading off some granularity here for a more comprehensible number. See, if you prepare 1000 queries, you are still doing 1000 roundtrips to the database, typically. But that’s a weird thing, and telling a developer how many “queries they did” is easier to understand when that means simply executing the query.

In line 2 we return ('query'). This is what causes all queries to be treated as if they were the same. We could have returned any constant string here. If we wanted to do something weird, like count based on type of query, we could do something clever like the following:

return (\undef) if $_[1] eq 'execute';
local $_ = $_;

return ($_);

That would create a bucket for SELECT, UPDATE, etc.

Ok back to dissection; line 3 returns (\undef), which is weird, but it’s how you signal that you do not want to include a given sample.

So the above is how you generate all of the profiling information. You can be more clever and include caller data or even bind parameters, though I’ll leave those as a post for another time. Additionally, you could carefully record your data and then do some kind of formatting at read time. Unlike DBIC_TRACE where you can end up with invalid SQL, you could use this with post-processing to show a formatted query if and only if it round trips.

Now go forth; record some performance information and ensure your app is fast!

UPDATE: I modified the ON_DESTROY_DUMP to set it to undef instead of an empty code reference. This correctly avoids a lot of work at object destriction time. Read this for more information.

Posted Thu, Mar 24, 2016