Disable prepared statements completely

Because of the way DBIx::Class does prepared statements, even
innocuous queries such

  $c->model('DB::Builds)->search({finished => 0})

can be extremely slow.  This is because DBIx::Class prepares a
PostgreSQL statement

  select ... from Builds where finished = ?

and since Builds is very large and there is a large fraction of rows
with "finished = 1", the PostgreSQL query planner decides to implement
this query with a sequential scan of the Builds table (despite the
existence of an index on "finished"), which is extremely slow.  It
would be nice if we could tell DBIx::Class that constants should be
part of the prepared statement, i.e.

  select ... from Builds where finished = 0

but AFAIK we can't.

src/lib/Hydra/Controller/Root.pm

@@ -95,15 +95,11 @@ sub timeline :Local {
 
 sub status :Local {
     my ($self, $c) = @_;
-    $c->model('DB')->storage->dbh_do(sub {
-        my (undef, $dbh) = @_;
-        local $dbh->{pg_server_prepare} = 0;
     $c->stash->{steps} = [ $c->model('DB::BuildSteps')->search(
         { 'me.busy' => 1, 'build.finished' => 0, 'build.busy' => 1 },
         { join => [ 'build' ]
         , order_by => [ 'machine' ]
         } ) ];
-    });
 }
 
 

src/lib/Hydra/Model/DB.pm

@@ -6,7 +6,10 @@ use Hydra::Helper::Nix;
 
 __PACKAGE__->config(
     schema_class => 'Hydra::Schema',
-    connect_info => [getHydraDBPath],
+    connect_info => {
+        dsn => getHydraDBPath,
+        pg_server_prepare => 0,
+    },
 );
 
 =head1 NAME