Materialize the nearby DB query for performance.

The easiest way to do this, given all the other query parameter
construction going on, seems to be fetch the finished query and
replace the table lookup with the materialized CTE of the data,
then look up the full rows after to save having to work out how
to construct them ourselves.

perllib/FixMyStreet/DB/ResultSet/Nearby.pm

@@ -28,30 +28,51 @@ sub nearby {
 
     FixMyStreet::DB::ResultSet::Problem->non_public_if_possible($params, $c, 'problem');
 
-    $rs = $c->cobrand->problems_restriction($rs);
-
     # Add in any optional extra query parameters
     $params = { %$params, %{$args{extra}} } if $args{extra};
 
     my $attrs = {
-        prefetch => { problem => [] },
+        join => 'problem',
         bind => [ $args{latitude}, $args{longitude}, $args{distance} ],
         order_by => [ 'distance', { -desc => 'created' } ],
         rows => $args{limit},
     };
+
+    # Construct the query, but do not run it
+    my $rs_with_restriction = $c->cobrand->problems_restriction($rs);
+    my $query = $rs_with_restriction->search( $params, $attrs )->as_query;
+    $query = $$query;
+
+    # Replace the table lookup with it being looked up in a CTE first, as that's much quicker
+    my $sql = shift @$query;
+    $sql =~ s/problem_find_nearby\(\?,\?,\?\) "me"/"me"/;
+    $sql = "WITH me AS MATERIALIZED ( SELECT * FROM problem_find_nearby( ?, ?, ? ) ) $sql";
+
+    # Now perform the query to get the right problem IDs in the right order
+    my $storage = $rs->result_source->storage;
+    my (undef, $sth, undef) = $storage->dbh_do( _dbh_execute => $sql, $query);
+    my $result = $sth->fetchall_arrayref;
+
+    $attrs = {};
     if ($c->user_exists) {
         if ($c->user->from_body || $c->user->is_superuser) {
-            push @{$attrs->{prefetch}{problem}}, 'contact';
+            push @{$attrs->{prefetch}}, 'contact';
         }
         if ($c->user->has_body_permission_to('planned_reports')) {
-            push @{$attrs->{prefetch}{problem}}, 'user_planned_reports';
+            push @{$attrs->{prefetch}}, 'user_planned_reports';
         }
         if ($c->user->has_body_permission_to('report_edit_priority') || $c->user->has_body_permission_to('report_inspect')) {
-            push @{$attrs->{prefetch}{problem}}, 'response_priority';
+            push @{$attrs->{prefetch}}, 'response_priority';
         }
     }
 
-    my @problems = mySociety::Locale::in_gb_locale { $rs->search( $params, $attrs )->all };
+    # Now look up the full rows for the relevant IDs fetched
+    my @ids = map { $_->[0] } @$result;
+    my @problems = $c->cobrand->problems->search( { 'me.id' => \@ids }, $attrs )->all;
+
+    # And construct Nearby rows as that's what the callers are expecting
+    my %problems = map { $_->id => $_ } @problems;
+    @problems = map { $rs->new_result({ problem => $problems{$_->[0]}, distance => $_->[1] }) } @$result;
     return \@problems;
 }